<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019ef5bf-f8f4-7013-aa54-d88a84576ae1,g:ec1b69b0-0062-426f-96de-02ffee37b94f-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/ConsolidatedStatementsofChangesinNetAssets" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#ConsolidatedStatementsofChangesinNetAssets"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/ConsolidatedStatementsofChangesinNetAssets" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7a56-8a06-048d82ab2192" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ef5c0-0bf9-73ef-be8d-9c6471ac587d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7a56-8a06-048d82ab2192" xlink:to="loc_us-gaap_StatementTable_019ef5c0-0bf9-73ef-be8d-9c6471ac587d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019ef5c0-0bf9-7bf1-a11c-94d53d35441e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-73ef-be8d-9c6471ac587d" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019ef5c0-0bf9-7bf1-a11c-94d53d35441e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ef5c0-0bf9-7bf1-a11c-94d53d35441e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ef5c0-0bf9-7bf1-a11c-94d53d35441e" xlink:to="loc_us-gaap_EquityComponentDomain_019ef5c0-0bf9-7bf1-a11c-94d53d35441e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019ef5c0-0bf9-7f61-9b99-df839e1df6b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019ef5c0-0bf9-7bf1-a11c-94d53d35441e" xlink:to="loc_us-gaap_EquityComponentDomain_019ef5c0-0bf9-7f61-9b99-df839e1df6b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019ef5c0-0bf9-7378-967a-9bc3231e1b68" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ef5c0-0bf9-7f61-9b99-df839e1df6b5" xlink:to="loc_us-gaap_CommonStockMember_019ef5c0-0bf9-7378-967a-9bc3231e1b68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019ef5c0-0bf9-7f86-952c-1f0c0bde15dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ef5c0-0bf9-7f61-9b99-df839e1df6b5" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019ef5c0-0bf9-7f86-952c-1f0c0bde15dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019ef5c0-0bf9-7d01-82a3-2ec01c0b5432" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019ef5c0-0bf9-7f61-9b99-df839e1df6b5" xlink:to="loc_us-gaap_RetainedEarningsMember_019ef5c0-0bf9-7d01-82a3-2ec01c0b5432" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyNetAssetsRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7a56-8a06-048d82ab2192" xlink:to="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ef5c0-0bf9-7673-ab6c-a0a02c3ae858" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019ef5c0-0bf9-7673-ab6c-a0a02c3ae858" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ef5c0-0bf9-7511-9833-598551284b65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_us-gaap_StockholdersEquity_019ef5c0-0bf9-7511-9833-598551284b65" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019ef5c0-0bf9-7d60-9991-523f489e889a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019ef5c0-0bf9-7d60-9991-523f489e889a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019ef5c0-0bf9-75d2-840a-b4b97263762d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019ef5c0-0bf9-75d2-840a-b4b97263762d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAbstract_019ef5c0-0bf9-7673-9518-d4be8a254a82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_us-gaap_NetIncomeLossAbstract_019ef5c0-0bf9-7673-9518-d4be8a254a82" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeOperatingAfterExpenseAndTax_019ef5c0-0bf9-75fc-8d30-3ec4f9f7da9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeOperatingAfterExpenseAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAbstract_019ef5c0-0bf9-7673-9518-d4be8a254a82" xlink:to="loc_us-gaap_InvestmentIncomeOperatingAfterExpenseAndTax_019ef5c0-0bf9-75fc-8d30-3ec4f9f7da9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax_019ef5c0-0bf9-7ad5-b813-4aced8b60f30" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAbstract_019ef5c0-0bf9-7673-9518-d4be8a254a82" xlink:to="loc_us-gaap_RealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax_019ef5c0-0bf9-7ad5-b813-4aced8b60f30" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax_019ef5c0-0bf9-7ac0-a7db-4089746c31b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAbstract_019ef5c0-0bf9-7673-9518-d4be8a254a82" xlink:to="loc_us-gaap_UnrealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax_019ef5c0-0bf9-7ac0-a7db-4089746c31b7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyDividendDistributionAbstract_019ef5c0-0bf9-7c26-b521-e1f875002837" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyDividendDistributionAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_us-gaap_InvestmentCompanyDividendDistributionAbstract_019ef5c0-0bf9-7c26-b521-e1f875002837" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan_019ef5c0-0bf9-7b00-a33e-ff6f9c0393b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyDividendDistributionAbstract_019ef5c0-0bf9-7c26-b521-e1f875002837" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan_019ef5c0-0bf9-7b00-a33e-ff6f9c0393b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan_019ef5c0-0bf9-724a-910f-92f0b823c0b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyDividendDistributionAbstract_019ef5c0-0bf9-7c26-b521-e1f875002837" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan_019ef5c0-0bf9-724a-910f-92f0b823c0b0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyDividendDistributionFromDistributableEarningsLosses_019ef5c0-0bf9-7fd0-801c-525ff2653e1d" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyDividendDistributionFromDistributableEarningsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyDividendDistributionAbstract_019ef5c0-0bf9-7c26-b521-e1f875002837" xlink:to="loc_gbdc4_InvestmentCompanyDividendDistributionFromDistributableEarningsLosses_019ef5c0-0bf9-7fd0-801c-525ff2653e1d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyDistributionDeclaredAndPayable_019ef5c0-0bf9-754e-8b19-f387cbed4e58" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyDistributionDeclaredAndPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyDividendDistributionAbstract_019ef5c0-0bf9-7c26-b521-e1f875002837" xlink:to="loc_gbdc4_InvestmentCompanyDistributionDeclaredAndPayable_019ef5c0-0bf9-754e-8b19-f387cbed4e58" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_TaxCutsAndJobsActReclassificationFromAPICToRetainedEarningsTaxEffect_019ef5c0-0bf9-7dd1-a356-41df0d7d4963" xlink:href="gbdc4-20250930.xsd#gbdc4_TaxCutsAndJobsActReclassificationFromAPICToRetainedEarningsTaxEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_gbdc4_TaxCutsAndJobsActReclassificationFromAPICToRetainedEarningsTaxEffect_019ef5c0-0bf9-7dd1-a356-41df0d7d4963" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesPeriodIncreaseDecrease_019ef5c0-0bf9-7d6c-9591-40e1d886e49a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesPeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesPeriodIncreaseDecrease_019ef5c0-0bf9-7d6c-9591-40e1d886e49a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityPeriodIncreaseDecrease_019ef5c0-0bf9-76df-97c3-a3a63be253d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityPeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyNetAssetsRollForward_019ef5c0-0bf9-700a-927e-65301dbea252" xlink:to="loc_us-gaap_StockholdersEquityPeriodIncreaseDecrease_019ef5c0-0bf9-76df-97c3-a3a63be253d9" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ef5c0-0bf9-7b83-8705-b87c7fd2683c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ef5c0-0bf9-7c20-ae41-c6da72fb0b2d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/ConsolidatedScheduleofInvestmentsunaudited" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#ConsolidatedScheduleofInvestmentsunaudited"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/ConsolidatedScheduleofInvestmentsunaudited" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIdentifierAxis_019ef5c0-0bf9-781e-ac40-0858f8ade4d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIdentifierAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:to="loc_us-gaap_InvestmentIdentifierAxis_019ef5c0-0bf9-781e-ac40-0858f8ade4d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bf9-762b-ad24-8a786961fa3e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesByIndustryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:to="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bf9-762b-ad24-8a786961fa3e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-762b-ad24-8a786961fa3e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bf9-762b-ad24-8a786961fa3e" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-762b-ad24-8a786961fa3e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bf9-762b-ad24-8a786961fa3e" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AerospaceAndDefenseSectorMember_019ef5c0-0bf9-7e42-8d61-133b6e1280d2" xlink:href="gbdc4-20250930.xsd#gbdc4_AerospaceAndDefenseSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_AerospaceAndDefenseSectorMember_019ef5c0-0bf9-7e42-8d61-133b6e1280d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AirFreightAndLogisticsMember_019ef5c0-0bf9-7065-b228-9c789a3b06f8" xlink:href="gbdc4-20250930.xsd#gbdc4_AirFreightAndLogisticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_AirFreightAndLogisticsMember_019ef5c0-0bf9-7065-b228-9c789a3b06f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AutoComponentsSectorMember_019ef5c0-0bf9-77ae-921c-f572168e9ff0" xlink:href="gbdc4-20250930.xsd#gbdc4_AutoComponentsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_AutoComponentsSectorMember_019ef5c0-0bf9-77ae-921c-f572168e9ff0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AutomobilesSectorMember_019ef5c0-0bf9-7c17-bb76-a21f4db2d70e" xlink:href="gbdc4-20250930.xsd#gbdc4_AutomobilesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_AutomobilesSectorMember_019ef5c0-0bf9-7c17-bb76-a21f4db2d70e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BanksSectorMember_019ef5c0-0bf9-7a39-be81-2f385d536447" xlink:href="gbdc4-20250930.xsd#gbdc4_BanksSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_BanksSectorMember_019ef5c0-0bf9-7a39-be81-2f385d536447" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BeveragesSectorMember_019ef5c0-0bf9-7dcd-a395-3b07c9ea0722" xlink:href="gbdc4-20250930.xsd#gbdc4_BeveragesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_BeveragesSectorMember_019ef5c0-0bf9-7dcd-a395-3b07c9ea0722" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BuildingProductsSectorMember_019ef5c0-0bf9-7473-95b3-10d953d95668" xlink:href="gbdc4-20250930.xsd#gbdc4_BuildingProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_BuildingProductsSectorMember_019ef5c0-0bf9-7473-95b3-10d953d95668" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CapitalMarketsSectorMember_019ef5c0-0bf9-7119-b2ea-fa0cae47665e" xlink:href="gbdc4-20250930.xsd#gbdc4_CapitalMarketsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_CapitalMarketsSectorMember_019ef5c0-0bf9-7119-b2ea-fa0cae47665e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChemicalsSectorMember_019ef5c0-0bf9-7320-b878-14c268a07a38" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChemicalsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_us-gaap_ChemicalsSectorMember_019ef5c0-0bf9-7320-b878-14c268a07a38" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CommercialServicesAndSuppliesSectorMember_019ef5c0-0bf9-752c-a47d-33e4b8a48a7d" xlink:href="gbdc4-20250930.xsd#gbdc4_CommercialServicesAndSuppliesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_CommercialServicesAndSuppliesSectorMember_019ef5c0-0bf9-752c-a47d-33e4b8a48a7d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ConstructionAndEngineeringSectorMember_019ef5c0-0bf9-73f3-be98-51654421947d" xlink:href="gbdc4-20250930.xsd#gbdc4_ConstructionAndEngineeringSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ConstructionAndEngineeringSectorMember_019ef5c0-0bf9-73f3-be98-51654421947d" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContainerAndPackagingSectorMember_019ef5c0-0bf9-732c-96ee-79d10f435342" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContainerAndPackagingSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_us-gaap_ContainerAndPackagingSectorMember_019ef5c0-0bf9-732c-96ee-79d10f435342" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DiversifiedConsumerServicesSectorMember_019ef5c0-0bf9-7c8d-9e3e-08f99761a78e" xlink:href="gbdc4-20250930.xsd#gbdc4_DiversifiedConsumerServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_DiversifiedConsumerServicesSectorMember_019ef5c0-0bf9-7c8d-9e3e-08f99761a78e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DiversifiedFinancialServicesSectorMember_019ef5c0-0bf9-73c1-9381-b7f119c60038" xlink:href="gbdc4-20250930.xsd#gbdc4_DiversifiedFinancialServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_DiversifiedFinancialServicesSectorMember_019ef5c0-0bf9-73c1-9381-b7f119c60038" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectricUtilitiesSectorMember_019ef5c0-0bf9-7f3d-9f22-636dee348813" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectricUtilitiesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ElectricUtilitiesSectorMember_019ef5c0-0bf9-7f3d-9f22-636dee348813" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectricalEquipmentSectorMember_019ef5c0-0bf9-7aa6-ad1e-8174647a2c22" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectricalEquipmentSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ElectricalEquipmentSectorMember_019ef5c0-0bf9-7aa6-ad1e-8174647a2c22" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember_019ef5c0-0bf9-7f34-b3ae-524a15053feb" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember_019ef5c0-0bf9-7f34-b3ae-524a15053feb" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_FoodProductsSectorMember_019ef5c0-0bf9-7545-926c-2aa41f317d5a" xlink:href="gbdc4-20250930.xsd#gbdc4_FoodProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_FoodProductsSectorMember_019ef5c0-0bf9-7545-926c-2aa41f317d5a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareEquipmentAndSuppliesSectorMember_019ef5c0-0bf9-7ec2-98ed-2db0b341cb8b" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareEquipmentAndSuppliesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HealthcareEquipmentAndSuppliesSectorMember_019ef5c0-0bf9-7ec2-98ed-2db0b341cb8b" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareProvidersAndServicesSectorMember_019ef5c0-0bf9-759e-b27b-dbba53dd2d31" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareProvidersAndServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HealthcareProvidersAndServicesSectorMember_019ef5c0-0bf9-759e-b27b-dbba53dd2d31" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareTechnologySectorMember_019ef5c0-0bf9-7ba0-851a-cc7bb3074496" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareTechnologySectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HealthcareTechnologySectorMember_019ef5c0-0bf9-7ba0-851a-cc7bb3074496" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HotelsRestaurantsAndLeisureSectorMember_019ef5c0-0bf9-7bac-9f48-b2b9de4d9866" xlink:href="gbdc4-20250930.xsd#gbdc4_HotelsRestaurantsAndLeisureSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HotelsRestaurantsAndLeisureSectorMember_019ef5c0-0bf9-7bac-9f48-b2b9de4d9866" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HouseholdProductsSectorMember_019ef5c0-0bf9-7ace-80b0-b405835cda6a" xlink:href="gbdc4-20250930.xsd#gbdc4_HouseholdProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HouseholdProductsSectorMember_019ef5c0-0bf9-7ace-80b0-b405835cda6a" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_IndustrialConglomeratesSectorMember_019ef5c0-0bf9-7284-89a9-2fd7d9768e94" xlink:href="gbdc4-20250930.xsd#gbdc4_IndustrialConglomeratesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_IndustrialConglomeratesSectorMember_019ef5c0-0bf9-7284-89a9-2fd7d9768e94" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InsuranceSectorMember_019ef5c0-0bf9-7831-a7ce-40e45e87e512" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InsuranceSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_us-gaap_InsuranceSectorMember_019ef5c0-0bf9-7831-a7ce-40e45e87e512" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ITServicesSectorMember_019ef5c0-0bf9-797f-88b6-028828b59458" xlink:href="gbdc4-20250930.xsd#gbdc4_ITServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ITServicesSectorMember_019ef5c0-0bf9-797f-88b6-028828b59458" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LeisureProductsSectorMember_019ef5c0-0bf9-73a3-accf-12583d916801" xlink:href="gbdc4-20250930.xsd#gbdc4_LeisureProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_LeisureProductsSectorMember_019ef5c0-0bf9-73a3-accf-12583d916801" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LifeSciencesToolsAndServicesSectorMember_019ef5c0-0bf9-72fb-889a-4145bf19e597" xlink:href="gbdc4-20250930.xsd#gbdc4_LifeSciencesToolsAndServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_LifeSciencesToolsAndServicesSectorMember_019ef5c0-0bf9-72fb-889a-4145bf19e597" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MachinerySectorMember_019ef5c0-0bf9-741c-a6d5-0ec27c69236e" xlink:href="gbdc4-20250930.xsd#gbdc4_MachinerySectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_MachinerySectorMember_019ef5c0-0bf9-741c-a6d5-0ec27c69236e" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MediaSectorMember_019ef5c0-0bf9-753c-8c28-1cdbdc6fd8f7" xlink:href="gbdc4-20250930.xsd#gbdc4_MediaSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_MediaSectorMember_019ef5c0-0bf9-753c-8c28-1cdbdc6fd8f7" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_OilGasAndConsumableFuelsSectorMember_019ef5c0-0bf9-7682-b15a-4759a2ce5db6" xlink:href="gbdc4-20250930.xsd#gbdc4_OilGasAndConsumableFuelsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_OilGasAndConsumableFuelsSectorMember_019ef5c0-0bf9-7682-b15a-4759a2ce5db6" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PharmaceuticalsSectorMember_019ef5c0-0bf9-74c2-b700-f6de529373bc" xlink:href="gbdc4-20250930.xsd#gbdc4_PharmaceuticalsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_PharmaceuticalsSectorMember_019ef5c0-0bf9-74c2-b700-f6de529373bc" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ProfessionalServicesSectorMember_019ef5c0-0bf9-711f-80d8-3e9cbffb33e6" xlink:href="gbdc4-20250930.xsd#gbdc4_ProfessionalServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ProfessionalServicesSectorMember_019ef5c0-0bf9-711f-80d8-3e9cbffb33e6" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_RoadRailSectorMember_019ef5c0-0bf9-75b9-b241-423a0b719572" xlink:href="gbdc4-20250930.xsd#gbdc4_RoadRailSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_RoadRailSectorMember_019ef5c0-0bf9-75b9-b241-423a0b719572" xlink:type="arc" order="33"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SoftwareSectorMember_019ef5c0-0bf9-72c8-bc69-dcaf910c20e7" xlink:href="gbdc4-20250930.xsd#gbdc4_SoftwareSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_SoftwareSectorMember_019ef5c0-0bf9-72c8-bc69-dcaf910c20e7" xlink:type="arc" order="34"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SpecialtyRetailSectorMember_019ef5c0-0bf9-7b3b-91fa-b59580f43784" xlink:href="gbdc4-20250930.xsd#gbdc4_SpecialtyRetailSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_SpecialtyRetailSectorMember_019ef5c0-0bf9-7b3b-91fa-b59580f43784" xlink:type="arc" order="35"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_TransportationInfrastructureSectorMember_019ef5c0-0bf9-7d21-94f8-b82321dc9aea" xlink:href="gbdc4-20250930.xsd#gbdc4_TransportationInfrastructureSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_TransportationInfrastructureSectorMember_019ef5c0-0bf9-7d21-94f8-b82321dc9aea" xlink:type="arc" order="36"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_WaterUtilitiesSectorMember_019ef5c0-0bf9-7b09-8b68-7f5467215c27" xlink:href="gbdc4-20250930.xsd#gbdc4_WaterUtilitiesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_WaterUtilitiesSectorMember_019ef5c0-0bf9-7b09-8b68-7f5467215c27" xlink:type="arc" order="37"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bf9-7198-86d9-b1d2db6c888d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:to="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bf9-7198-86d9-b1d2db6c888d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-7198-86d9-b1d2db6c888d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bf9-7198-86d9-b1d2db6c888d" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-7198-86d9-b1d2db6c888d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-778a-80a8-be11d7915381" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bf9-7198-86d9-b1d2db6c888d" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-778a-80a8-be11d7915381" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesMember_019ef5c0-0bf9-7974-a4e5-169db7822249" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-778a-80a8-be11d7915381" xlink:to="loc_us-gaap_DebtSecuritiesMember_019ef5c0-0bf9-7974-a4e5-169db7822249" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bf9-76c9-b6bc-b24880f31de0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-778a-80a8-be11d7915381" xlink:to="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bf9-76c9-b6bc-b24880f31de0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentBasisSpreadVariableRate_019ef5c0-0bf9-7e62-85fc-a6f16094126f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentBasisSpreadVariableRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentBasisSpreadVariableRate_019ef5c0-0bf9-7e62-85fc-a6f16094126f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentInterestRate_019ef5c0-0bf9-7e88-a60d-69154e074a74" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentInterestRate_019ef5c0-0bf9-7e88-a60d-69154e074a74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentInterestRatePaidInKind_019ef5c0-0bf9-72e8-a336-0d8ec051e2b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentInterestRatePaidInKind"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentInterestRatePaidInKind_019ef5c0-0bf9-72e8-a336-0d8ec051e2b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedBalancePrincipalAmount_019ef5c0-0bf9-75ee-871d-7d51aacce298" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedBalancePrincipalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedBalancePrincipalAmount_019ef5c0-0bf9-75ee-871d-7d51aacce298" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedBalanceShares_019ef5c0-0bf9-763c-ba8f-b38ed56f304d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedBalanceShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedBalanceShares_019ef5c0-0bf9-763c-ba8f-b38ed56f304d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bf9-77b8-bff4-492d7e73c6d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bf9-77b8-bff4-492d7e73c6d4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedPercentOfNetAssets_019ef5c0-0bf9-76de-9f1b-c32ee2f08c99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedPercentOfNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedPercentOfNetAssets_019ef5c0-0bf9-76de-9f1b-c32ee2f08c99" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bf9-7535-868c-8497e3ccc1f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bf9-7535-868c-8497e3ccc1f4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-74a0-9d79-6c1c1b277769" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_MoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-74a0-9d79-6c1c1b277769" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MoneyMarketFundsPercentOfNetAssets_019ef5c0-0bf9-7246-8a92-6f73c0ddc84a" xlink:href="gbdc4-20250930.xsd#gbdc4_MoneyMarketFundsPercentOfNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_gbdc4_MoneyMarketFundsPercentOfNetAssets_019ef5c0-0bf9-7246-8a92-6f73c0ddc84a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentOwnedAtCostAndMoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-7e80-b7de-dcc27f3682e5" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentOwnedAtCostAndMoneyMarketFundsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_gbdc4_InvestmentOwnedAtCostAndMoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-7e80-b7de-dcc27f3682e5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentOwnedAndMoneyMarketFundsPercentOfNetAssets_019ef5c0-0bf9-713b-867d-b485feff59b0" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentOwnedAndMoneyMarketFundsPercentOfNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_gbdc4_InvestmentOwnedAndMoneyMarketFundsPercentOfNetAssets_019ef5c0-0bf9-713b-867d-b485feff59b0" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentOwnedAtFairValueAndMoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-76f8-912c-f4b35c5d35ce" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentOwnedAtFairValueAndMoneyMarketFundsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_gbdc4_InvestmentOwnedAtFairValueAndMoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-76f8-912c-f4b35c5d35ce" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/ConsolidatedScheduleofInvestmentsunaudited_1" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#ConsolidatedScheduleofInvestmentsunaudited_1"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/ConsolidatedScheduleofInvestmentsunaudited_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bf9-762b-ad24-8a786961fa3e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesByIndustryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:to="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bf9-762b-ad24-8a786961fa3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-762b-ad24-8a786961fa3e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bf9-762b-ad24-8a786961fa3e" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-762b-ad24-8a786961fa3e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bf9-762b-ad24-8a786961fa3e" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AerospaceAndDefenseSectorMember_019ef5c0-0bf9-7e42-8d61-133b6e1280d2" xlink:href="gbdc4-20250930.xsd#gbdc4_AerospaceAndDefenseSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_AerospaceAndDefenseSectorMember_019ef5c0-0bf9-7e42-8d61-133b6e1280d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AirFreightAndLogisticsMember_019ef5c0-0bf9-7065-b228-9c789a3b06f8" xlink:href="gbdc4-20250930.xsd#gbdc4_AirFreightAndLogisticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_AirFreightAndLogisticsMember_019ef5c0-0bf9-7065-b228-9c789a3b06f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AutoComponentsSectorMember_019ef5c0-0bf9-77ae-921c-f572168e9ff0" xlink:href="gbdc4-20250930.xsd#gbdc4_AutoComponentsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_AutoComponentsSectorMember_019ef5c0-0bf9-77ae-921c-f572168e9ff0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AutomobilesSectorMember_019ef5c0-0bf9-7c17-bb76-a21f4db2d70e" xlink:href="gbdc4-20250930.xsd#gbdc4_AutomobilesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_AutomobilesSectorMember_019ef5c0-0bf9-7c17-bb76-a21f4db2d70e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BanksSectorMember_019ef5c0-0bf9-7a39-be81-2f385d536447" xlink:href="gbdc4-20250930.xsd#gbdc4_BanksSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_BanksSectorMember_019ef5c0-0bf9-7a39-be81-2f385d536447" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BeveragesSectorMember_019ef5c0-0bf9-7dcd-a395-3b07c9ea0722" xlink:href="gbdc4-20250930.xsd#gbdc4_BeveragesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_BeveragesSectorMember_019ef5c0-0bf9-7dcd-a395-3b07c9ea0722" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BuildingProductsSectorMember_019ef5c0-0bf9-7473-95b3-10d953d95668" xlink:href="gbdc4-20250930.xsd#gbdc4_BuildingProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_BuildingProductsSectorMember_019ef5c0-0bf9-7473-95b3-10d953d95668" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CapitalMarketsSectorMember_019ef5c0-0bf9-7119-b2ea-fa0cae47665e" xlink:href="gbdc4-20250930.xsd#gbdc4_CapitalMarketsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_CapitalMarketsSectorMember_019ef5c0-0bf9-7119-b2ea-fa0cae47665e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChemicalsSectorMember_019ef5c0-0bf9-7320-b878-14c268a07a38" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChemicalsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_us-gaap_ChemicalsSectorMember_019ef5c0-0bf9-7320-b878-14c268a07a38" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CommercialServicesAndSuppliesSectorMember_019ef5c0-0bf9-752c-a47d-33e4b8a48a7d" xlink:href="gbdc4-20250930.xsd#gbdc4_CommercialServicesAndSuppliesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_CommercialServicesAndSuppliesSectorMember_019ef5c0-0bf9-752c-a47d-33e4b8a48a7d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ConstructionAndEngineeringSectorMember_019ef5c0-0bf9-73f3-be98-51654421947d" xlink:href="gbdc4-20250930.xsd#gbdc4_ConstructionAndEngineeringSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ConstructionAndEngineeringSectorMember_019ef5c0-0bf9-73f3-be98-51654421947d" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContainerAndPackagingSectorMember_019ef5c0-0bf9-732c-96ee-79d10f435342" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContainerAndPackagingSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_us-gaap_ContainerAndPackagingSectorMember_019ef5c0-0bf9-732c-96ee-79d10f435342" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DiversifiedConsumerServicesSectorMember_019ef5c0-0bf9-7c8d-9e3e-08f99761a78e" xlink:href="gbdc4-20250930.xsd#gbdc4_DiversifiedConsumerServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_DiversifiedConsumerServicesSectorMember_019ef5c0-0bf9-7c8d-9e3e-08f99761a78e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DiversifiedFinancialServicesSectorMember_019ef5c0-0bf9-73c1-9381-b7f119c60038" xlink:href="gbdc4-20250930.xsd#gbdc4_DiversifiedFinancialServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_DiversifiedFinancialServicesSectorMember_019ef5c0-0bf9-73c1-9381-b7f119c60038" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectricUtilitiesSectorMember_019ef5c0-0bf9-7f3d-9f22-636dee348813" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectricUtilitiesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ElectricUtilitiesSectorMember_019ef5c0-0bf9-7f3d-9f22-636dee348813" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectricalEquipmentSectorMember_019ef5c0-0bf9-7aa6-ad1e-8174647a2c22" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectricalEquipmentSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ElectricalEquipmentSectorMember_019ef5c0-0bf9-7aa6-ad1e-8174647a2c22" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember_019ef5c0-0bf9-7f34-b3ae-524a15053feb" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember_019ef5c0-0bf9-7f34-b3ae-524a15053feb" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_FoodProductsSectorMember_019ef5c0-0bf9-7545-926c-2aa41f317d5a" xlink:href="gbdc4-20250930.xsd#gbdc4_FoodProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_FoodProductsSectorMember_019ef5c0-0bf9-7545-926c-2aa41f317d5a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareEquipmentAndSuppliesSectorMember_019ef5c0-0bf9-7ec2-98ed-2db0b341cb8b" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareEquipmentAndSuppliesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HealthcareEquipmentAndSuppliesSectorMember_019ef5c0-0bf9-7ec2-98ed-2db0b341cb8b" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareProvidersAndServicesSectorMember_019ef5c0-0bf9-759e-b27b-dbba53dd2d31" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareProvidersAndServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HealthcareProvidersAndServicesSectorMember_019ef5c0-0bf9-759e-b27b-dbba53dd2d31" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareTechnologySectorMember_019ef5c0-0bf9-7ba0-851a-cc7bb3074496" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareTechnologySectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HealthcareTechnologySectorMember_019ef5c0-0bf9-7ba0-851a-cc7bb3074496" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HotelsRestaurantsAndLeisureSectorMember_019ef5c0-0bf9-7bac-9f48-b2b9de4d9866" xlink:href="gbdc4-20250930.xsd#gbdc4_HotelsRestaurantsAndLeisureSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HotelsRestaurantsAndLeisureSectorMember_019ef5c0-0bf9-7bac-9f48-b2b9de4d9866" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HouseholdProductsSectorMember_019ef5c0-0bf9-7ace-80b0-b405835cda6a" xlink:href="gbdc4-20250930.xsd#gbdc4_HouseholdProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_HouseholdProductsSectorMember_019ef5c0-0bf9-7ace-80b0-b405835cda6a" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_IndustrialConglomeratesSectorMember_019ef5c0-0bf9-7284-89a9-2fd7d9768e94" xlink:href="gbdc4-20250930.xsd#gbdc4_IndustrialConglomeratesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_IndustrialConglomeratesSectorMember_019ef5c0-0bf9-7284-89a9-2fd7d9768e94" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InsuranceSectorMember_019ef5c0-0bf9-7831-a7ce-40e45e87e512" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InsuranceSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_us-gaap_InsuranceSectorMember_019ef5c0-0bf9-7831-a7ce-40e45e87e512" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ITServicesSectorMember_019ef5c0-0bf9-797f-88b6-028828b59458" xlink:href="gbdc4-20250930.xsd#gbdc4_ITServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ITServicesSectorMember_019ef5c0-0bf9-797f-88b6-028828b59458" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LeisureProductsSectorMember_019ef5c0-0bf9-73a3-accf-12583d916801" xlink:href="gbdc4-20250930.xsd#gbdc4_LeisureProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_LeisureProductsSectorMember_019ef5c0-0bf9-73a3-accf-12583d916801" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LifeSciencesToolsAndServicesSectorMember_019ef5c0-0bf9-72fb-889a-4145bf19e597" xlink:href="gbdc4-20250930.xsd#gbdc4_LifeSciencesToolsAndServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_LifeSciencesToolsAndServicesSectorMember_019ef5c0-0bf9-72fb-889a-4145bf19e597" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MachinerySectorMember_019ef5c0-0bf9-741c-a6d5-0ec27c69236e" xlink:href="gbdc4-20250930.xsd#gbdc4_MachinerySectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_MachinerySectorMember_019ef5c0-0bf9-741c-a6d5-0ec27c69236e" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MediaSectorMember_019ef5c0-0bf9-753c-8c28-1cdbdc6fd8f7" xlink:href="gbdc4-20250930.xsd#gbdc4_MediaSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_MediaSectorMember_019ef5c0-0bf9-753c-8c28-1cdbdc6fd8f7" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_OilGasAndConsumableFuelsSectorMember_019ef5c0-0bf9-7682-b15a-4759a2ce5db6" xlink:href="gbdc4-20250930.xsd#gbdc4_OilGasAndConsumableFuelsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_OilGasAndConsumableFuelsSectorMember_019ef5c0-0bf9-7682-b15a-4759a2ce5db6" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PharmaceuticalsSectorMember_019ef5c0-0bf9-74c2-b700-f6de529373bc" xlink:href="gbdc4-20250930.xsd#gbdc4_PharmaceuticalsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_PharmaceuticalsSectorMember_019ef5c0-0bf9-74c2-b700-f6de529373bc" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ProfessionalServicesSectorMember_019ef5c0-0bf9-711f-80d8-3e9cbffb33e6" xlink:href="gbdc4-20250930.xsd#gbdc4_ProfessionalServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_ProfessionalServicesSectorMember_019ef5c0-0bf9-711f-80d8-3e9cbffb33e6" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_RoadRailSectorMember_019ef5c0-0bf9-75b9-b241-423a0b719572" xlink:href="gbdc4-20250930.xsd#gbdc4_RoadRailSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_RoadRailSectorMember_019ef5c0-0bf9-75b9-b241-423a0b719572" xlink:type="arc" order="33"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SoftwareSectorMember_019ef5c0-0bf9-72c8-bc69-dcaf910c20e7" xlink:href="gbdc4-20250930.xsd#gbdc4_SoftwareSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_SoftwareSectorMember_019ef5c0-0bf9-72c8-bc69-dcaf910c20e7" xlink:type="arc" order="34"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SpecialtyRetailSectorMember_019ef5c0-0bf9-7b3b-91fa-b59580f43784" xlink:href="gbdc4-20250930.xsd#gbdc4_SpecialtyRetailSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_SpecialtyRetailSectorMember_019ef5c0-0bf9-7b3b-91fa-b59580f43784" xlink:type="arc" order="35"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_TransportationInfrastructureSectorMember_019ef5c0-0bf9-7d21-94f8-b82321dc9aea" xlink:href="gbdc4-20250930.xsd#gbdc4_TransportationInfrastructureSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_TransportationInfrastructureSectorMember_019ef5c0-0bf9-7d21-94f8-b82321dc9aea" xlink:type="arc" order="36"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_WaterUtilitiesSectorMember_019ef5c0-0bf9-7b09-8b68-7f5467215c27" xlink:href="gbdc4-20250930.xsd#gbdc4_WaterUtilitiesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bf9-7287-82c8-7481c70d3d84" xlink:to="loc_gbdc4_WaterUtilitiesSectorMember_019ef5c0-0bf9-7b09-8b68-7f5467215c27" xlink:type="arc" order="37"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bf9-7198-86d9-b1d2db6c888d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-79fe-8339-f05ee90b610e" xlink:to="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bf9-7198-86d9-b1d2db6c888d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-7198-86d9-b1d2db6c888d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bf9-7198-86d9-b1d2db6c888d" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-7198-86d9-b1d2db6c888d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-778a-80a8-be11d7915381" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bf9-7198-86d9-b1d2db6c888d" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-778a-80a8-be11d7915381" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesMember_019ef5c0-0bf9-7974-a4e5-169db7822249" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-778a-80a8-be11d7915381" xlink:to="loc_us-gaap_DebtSecuritiesMember_019ef5c0-0bf9-7974-a4e5-169db7822249" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bf9-76c9-b6bc-b24880f31de0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bf9-778a-80a8-be11d7915381" xlink:to="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bf9-76c9-b6bc-b24880f31de0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentBasisSpreadVariableRate_019ef5c0-0bf9-7e62-85fc-a6f16094126f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentBasisSpreadVariableRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentBasisSpreadVariableRate_019ef5c0-0bf9-7e62-85fc-a6f16094126f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentInterestRate_019ef5c0-0bf9-7e88-a60d-69154e074a74" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentInterestRate_019ef5c0-0bf9-7e88-a60d-69154e074a74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentInterestRatePaidInKind_019ef5c0-0bf9-72e8-a336-0d8ec051e2b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentInterestRatePaidInKind"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentInterestRatePaidInKind_019ef5c0-0bf9-72e8-a336-0d8ec051e2b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedBalancePrincipalAmount_019ef5c0-0bf9-75ee-871d-7d51aacce298" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedBalancePrincipalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedBalancePrincipalAmount_019ef5c0-0bf9-75ee-871d-7d51aacce298" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedBalanceShares_019ef5c0-0bf9-763c-ba8f-b38ed56f304d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedBalanceShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedBalanceShares_019ef5c0-0bf9-763c-ba8f-b38ed56f304d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bf9-77b8-bff4-492d7e73c6d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bf9-77b8-bff4-492d7e73c6d4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedPercentOfNetAssets_019ef5c0-0bf9-76de-9f1b-c32ee2f08c99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedPercentOfNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedPercentOfNetAssets_019ef5c0-0bf9-76de-9f1b-c32ee2f08c99" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bf9-7535-868c-8497e3ccc1f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bf9-7535-868c-8497e3ccc1f4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-74a0-9d79-6c1c1b277769" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_us-gaap_MoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-74a0-9d79-6c1c1b277769" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MoneyMarketFundsPercentOfNetAssets_019ef5c0-0bf9-7246-8a92-6f73c0ddc84a" xlink:href="gbdc4-20250930.xsd#gbdc4_MoneyMarketFundsPercentOfNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_gbdc4_MoneyMarketFundsPercentOfNetAssets_019ef5c0-0bf9-7246-8a92-6f73c0ddc84a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentOwnedAtCostAndMoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-7e80-b7de-dcc27f3682e5" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentOwnedAtCostAndMoneyMarketFundsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_gbdc4_InvestmentOwnedAtCostAndMoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-7e80-b7de-dcc27f3682e5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentOwnedAndMoneyMarketFundsPercentOfNetAssets_019ef5c0-0bf9-713b-867d-b485feff59b0" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentOwnedAndMoneyMarketFundsPercentOfNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_gbdc4_InvestmentOwnedAndMoneyMarketFundsPercentOfNetAssets_019ef5c0-0bf9-713b-867d-b485feff59b0" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentOwnedAtFairValueAndMoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-76f8-912c-f4b35c5d35ce" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentOwnedAtFairValueAndMoneyMarketFundsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-7c92-991a-2917fd11b80a" xlink:to="loc_gbdc4_InvestmentOwnedAtFairValueAndMoneyMarketFundsAtCarryingValue_019ef5c0-0bf9-76f8-912c-f4b35c5d35ce" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/ConsolidatedScheduleofInvestmentsunauditedParenthetical" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#ConsolidatedScheduleofInvestmentsunauditedParenthetical"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/ConsolidatedScheduleofInvestmentsunauditedParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-73b2-84cc-9d5c731f72d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019ef5c0-0bf9-7d48-bbf8-6d1679011282" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-73b2-84cc-9d5c731f72d1" xlink:to="loc_us-gaap_StatementTable_019ef5c0-0bf9-7d48-bbf8-6d1679011282" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_QualifyingAssetTypeAxis_019ef5c0-0bf9-7cf8-90a3-b9c58adead8c" xlink:href="gbdc4-20250930.xsd#gbdc4_QualifyingAssetTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-7d48-bbf8-6d1679011282" xlink:to="loc_gbdc4_QualifyingAssetTypeAxis_019ef5c0-0bf9-7cf8-90a3-b9c58adead8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_QualifyingAssetTypeDomain_019ef5c0-0bf9-7cf8-90a3-b9c58adead8c_default" xlink:href="gbdc4-20250930.xsd#gbdc4_QualifyingAssetTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_gbdc4_QualifyingAssetTypeAxis_019ef5c0-0bf9-7cf8-90a3-b9c58adead8c" xlink:to="loc_gbdc4_QualifyingAssetTypeDomain_019ef5c0-0bf9-7cf8-90a3-b9c58adead8c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_QualifyingAssetTypeDomain_019ef5c0-0bf9-717d-bce3-a2895290a27f" xlink:href="gbdc4-20250930.xsd#gbdc4_QualifyingAssetTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_gbdc4_QualifyingAssetTypeAxis_019ef5c0-0bf9-7cf8-90a3-b9c58adead8c" xlink:to="loc_gbdc4_QualifyingAssetTypeDomain_019ef5c0-0bf9-717d-bce3-a2895290a27f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_NonQualifyingAssetMember_019ef5c0-0bf9-7b94-9ecc-a065dfb901b3" xlink:href="gbdc4-20250930.xsd#gbdc4_NonQualifyingAssetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_QualifyingAssetTypeDomain_019ef5c0-0bf9-717d-bce3-a2895290a27f" xlink:to="loc_gbdc4_NonQualifyingAssetMember_019ef5c0-0bf9-7b94-9ecc-a065dfb901b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bf9-7244-82ed-d10aa87bf1a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-7d48-bbf8-6d1679011282" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bf9-7244-82ed-d10aa87bf1a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bf9-7244-82ed-d10aa87bf1a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bf9-7244-82ed-d10aa87bf1a6" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bf9-7244-82ed-d10aa87bf1a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bf9-7cd7-ab3e-05c4d695994b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bf9-7244-82ed-d10aa87bf1a6" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bf9-7cd7-ab3e-05c4d695994b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentsAtFairValueMember_019ef5c0-0bf9-7148-bf79-cf93f346f37d" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentsAtFairValueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bf9-7cd7-ab3e-05c4d695994b" xlink:to="loc_gbdc4_InvestmentsAtFairValueMember_019ef5c0-0bf9-7148-bf79-cf93f346f37d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bf9-7a8f-899c-cdf4adc60225" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019ef5c0-0bf9-7d48-bbf8-6d1679011282" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bf9-7a8f-899c-cdf4adc60225" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bf9-7a8f-899c-cdf4adc60225_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bf9-7a8f-899c-cdf4adc60225" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bf9-7a8f-899c-cdf4adc60225_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bf9-7ecd-9e81-322eadf791d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bf9-7a8f-899c-cdf4adc60225" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bf9-7ecd-9e81-322eadf791d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AssetConcentrationRiskMember_019ef5c0-0bf9-72ef-a74b-a019f3c80635" xlink:href="gbdc4-20250930.xsd#gbdc4_AssetConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bf9-7ecd-9e81-322eadf791d8" xlink:to="loc_gbdc4_AssetConcentrationRiskMember_019ef5c0-0bf9-72ef-a74b-a019f3c80635" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019ef5c0-0bf9-727d-93a7-2dbbe704e24d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019ef5c0-0bf9-73b2-84cc-9d5c731f72d1" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019ef5c0-0bf9-727d-93a7-2dbbe704e24d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/RelatedPartyTransactionsDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#RelatedPartyTransactionsDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/RelatedPartyTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_019ef5c0-0bf9-7b22-b4df-9ba43a9e891a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_019ef5c0-0bf9-7b22-b4df-9ba43a9e891a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b22-b4df-9ba43a9e891a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019ef5c0-0bf9-7b22-b4df-9ba43a9e891a" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b22-b4df-9ba43a9e891a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b40-af0c-fe2bba2810d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019ef5c0-0bf9-7b22-b4df-9ba43a9e891a" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b40-af0c-fe2bba2810d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentAdvisoryAgreementMember_019ef5c0-0bf9-7c7f-9852-9b5165611ca8" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentAdvisoryAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b40-af0c-fe2bba2810d7" xlink:to="loc_gbdc4_InvestmentAdvisoryAgreementMember_019ef5c0-0bf9-7c7f-9852-9b5165611ca8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentManagementAgreementIncentiveRateQuarterlyHurdleRateMember_019ef5c0-0bf9-70d3-940e-c5197b50f57b" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentManagementAgreementIncentiveRateQuarterlyHurdleRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_InvestmentAdvisoryAgreementMember_019ef5c0-0bf9-7c7f-9852-9b5165611ca8" xlink:to="loc_gbdc4_InvestmentManagementAgreementIncentiveRateQuarterlyHurdleRateMember_019ef5c0-0bf9-70d3-940e-c5197b50f57b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentManagementAgreementQuarterlyIncentiveRateCriteriaNotMetMember_019ef5c0-0bf9-701a-8700-cbc287d9af7f" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentManagementAgreementQuarterlyIncentiveRateCriteriaNotMetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_InvestmentAdvisoryAgreementMember_019ef5c0-0bf9-7c7f-9852-9b5165611ca8" xlink:to="loc_gbdc4_InvestmentManagementAgreementQuarterlyIncentiveRateCriteriaNotMetMember_019ef5c0-0bf9-701a-8700-cbc287d9af7f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentManagementAgreementIncentiveRatePreIncentiveFeeNetInvestmentIncomeBelowCatchUpMember_019ef5c0-0bf9-7c1b-890e-19ab0281c583" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentManagementAgreementIncentiveRatePreIncentiveFeeNetInvestmentIncomeBelowCatchUpMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_InvestmentAdvisoryAgreementMember_019ef5c0-0bf9-7c7f-9852-9b5165611ca8" xlink:to="loc_gbdc4_InvestmentManagementAgreementIncentiveRatePreIncentiveFeeNetInvestmentIncomeBelowCatchUpMember_019ef5c0-0bf9-7c1b-890e-19ab0281c583" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentAdvisoryAgreementCumulativePreIncentiveFeeNetInvestmentIncomeMember_019ef5c0-0bf9-7f54-91b5-d65f750888a6" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentAdvisoryAgreementCumulativePreIncentiveFeeNetInvestmentIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_InvestmentAdvisoryAgreementMember_019ef5c0-0bf9-7c7f-9852-9b5165611ca8" xlink:to="loc_gbdc4_InvestmentAdvisoryAgreementCumulativePreIncentiveFeeNetInvestmentIncomeMember_019ef5c0-0bf9-7f54-91b5-d65f750888a6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentAdvisoryAgreementCapitalGainIncentiveFeeBaseMember_019ef5c0-0bf9-7163-99f1-57a1a5598341" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentAdvisoryAgreementCapitalGainIncentiveFeeBaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_InvestmentAdvisoryAgreementMember_019ef5c0-0bf9-7c7f-9852-9b5165611ca8" xlink:to="loc_gbdc4_InvestmentAdvisoryAgreementCapitalGainIncentiveFeeBaseMember_019ef5c0-0bf9-7163-99f1-57a1a5598341" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentAdvisoryAgreementSubordinatedLiquidationIncentiveFeeBaseMember_019ef5c0-0bf9-7d03-8709-df53323b89b6" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentAdvisoryAgreementSubordinatedLiquidationIncentiveFeeBaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_InvestmentAdvisoryAgreementMember_019ef5c0-0bf9-7c7f-9852-9b5165611ca8" xlink:to="loc_gbdc4_InvestmentAdvisoryAgreementSubordinatedLiquidationIncentiveFeeBaseMember_019ef5c0-0bf9-7d03-8709-df53323b89b6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AdministrationAgreementMember_019ef5c0-0bf9-7545-b6ae-1e95143fb13d" xlink:href="gbdc4-20250930.xsd#gbdc4_AdministrationAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b40-af0c-fe2bba2810d7" xlink:to="loc_gbdc4_AdministrationAgreementMember_019ef5c0-0bf9-7545-b6ae-1e95143fb13d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentAdviserMember_019ef5c0-0bf9-71c5-ba52-70b7451202fb" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentAdviserMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b40-af0c-fe2bba2810d7" xlink:to="loc_gbdc4_InvestmentAdviserMember_019ef5c0-0bf9-71c5-ba52-70b7451202fb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ExpensesPaidMember_019ef5c0-0bf9-7497-a841-8edabf8539d2" xlink:href="gbdc4-20250930.xsd#gbdc4_ExpensesPaidMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b40-af0c-fe2bba2810d7" xlink:to="loc_gbdc4_ExpensesPaidMember_019ef5c0-0bf9-7497-a841-8edabf8539d2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ConversionToMarylandCorporationAcquisitionOfStockMember_019ef5c0-0bf9-71aa-8270-86ca248a6ccb" xlink:href="gbdc4-20250930.xsd#gbdc4_ConversionToMarylandCorporationAcquisitionOfStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019ef5c0-0bf9-7b40-af0c-fe2bba2810d7" xlink:to="loc_gbdc4_ConversionToMarylandCorporationAcquisitionOfStockMember_019ef5c0-0bf9-71aa-8270-86ca248a6ccb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ef5c0-0bf9-78b9-a8ad-39f510035e5d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ef5c0-0bf9-78b9-a8ad-39f510035e5d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019ef5c0-0bf9-78b9-a8ad-39f510035e5d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ef5c0-0bf9-78b9-a8ad-39f510035e5d" xlink:to="loc_us-gaap_RelatedPartyDomain_019ef5c0-0bf9-78b9-a8ad-39f510035e5d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019ef5c0-0bf9-79c1-a913-85fcbef61421" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019ef5c0-0bf9-78b9-a8ad-39f510035e5d" xlink:to="loc_us-gaap_RelatedPartyDomain_019ef5c0-0bf9-79c1-a913-85fcbef61421" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019ef5c0-0bf9-7b1c-937f-54b8c6e9a2af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019ef5c0-0bf9-79c1-a913-85fcbef61421" xlink:to="loc_us-gaap_RelatedPartyMember_019ef5c0-0bf9-7b1c-937f-54b8c6e9a2af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_IncentiveFeeScenarioAxis_019ef5c0-0bf9-726c-8d85-6bf85740ead6" xlink:href="gbdc4-20250930.xsd#gbdc4_IncentiveFeeScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:to="loc_gbdc4_IncentiveFeeScenarioAxis_019ef5c0-0bf9-726c-8d85-6bf85740ead6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_IncentiveFeeScenarioDomain_019ef5c0-0bf9-726c-8d85-6bf85740ead6_default" xlink:href="gbdc4-20250930.xsd#gbdc4_IncentiveFeeScenarioDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_gbdc4_IncentiveFeeScenarioAxis_019ef5c0-0bf9-726c-8d85-6bf85740ead6" xlink:to="loc_gbdc4_IncentiveFeeScenarioDomain_019ef5c0-0bf9-726c-8d85-6bf85740ead6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_IncentiveFeeScenarioDomain_019ef5c0-0bf9-7b31-a85b-31b10be0fc7b" xlink:href="gbdc4-20250930.xsd#gbdc4_IncentiveFeeScenarioDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_gbdc4_IncentiveFeeScenarioAxis_019ef5c0-0bf9-726c-8d85-6bf85740ead6" xlink:to="loc_gbdc4_IncentiveFeeScenarioDomain_019ef5c0-0bf9-7b31-a85b-31b10be0fc7b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PeriodsEndingOnOrPriorToTheDateOfTheClosingOfTheLiquidityEventMember_019ef5c0-0bf9-7c6d-b9bd-59d9d977fa64" xlink:href="gbdc4-20250930.xsd#gbdc4_PeriodsEndingOnOrPriorToTheDateOfTheClosingOfTheLiquidityEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_IncentiveFeeScenarioDomain_019ef5c0-0bf9-7b31-a85b-31b10be0fc7b" xlink:to="loc_gbdc4_PeriodsEndingOnOrPriorToTheDateOfTheClosingOfTheLiquidityEventMember_019ef5c0-0bf9-7c6d-b9bd-59d9d977fa64" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PeriodsEndingAfterTheDateOfTheClosingOfTheLiquidityEventMember_019ef5c0-0bf9-75d6-b4ed-62bc81a485cd" xlink:href="gbdc4-20250930.xsd#gbdc4_PeriodsEndingAfterTheDateOfTheClosingOfTheLiquidityEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_IncentiveFeeScenarioDomain_019ef5c0-0bf9-7b31-a85b-31b10be0fc7b" xlink:to="loc_gbdc4_PeriodsEndingAfterTheDateOfTheClosingOfTheLiquidityEventMember_019ef5c0-0bf9-75d6-b4ed-62bc81a485cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019ef5c0-0bf9-7843-94c6-627a43f4445c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:to="loc_srt_CounterpartyNameAxis_019ef5c0-0bf9-7843-94c6-627a43f4445c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bf9-7843-94c6-627a43f4445c_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019ef5c0-0bf9-7843-94c6-627a43f4445c" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bf9-7843-94c6-627a43f4445c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bf9-7bb9-8553-dfddef9c67fc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019ef5c0-0bf9-7843-94c6-627a43f4445c" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bf9-7bb9-8553-dfddef9c67fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_GGPHoldingsLPMember_019ef5c0-0bf9-7066-97d3-aa29ff362e33" xlink:href="gbdc4-20250930.xsd#gbdc4_GGPHoldingsLPMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bf9-7bb9-8553-dfddef9c67fc" xlink:to="loc_gbdc4_GGPHoldingsLPMember_019ef5c0-0bf9-7066-97d3-aa29ff362e33" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_GGPClassBPLLCMember_019ef5c0-0bf9-739d-aedc-2241d798a543" xlink:href="gbdc4-20250930.xsd#gbdc4_GGPClassBPLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bf9-7bb9-8553-dfddef9c67fc" xlink:to="loc_gbdc4_GGPClassBPLLCMember_019ef5c0-0bf9-739d-aedc-2241d798a543" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019ef5c0-0bf9-7920-a1e6-5ac218ed2fea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019ef5c0-0bf9-7920-a1e6-5ac218ed2fea" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ef5c0-0bf9-7920-a1e6-5ac218ed2fea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ef5c0-0bf9-7920-a1e6-5ac218ed2fea" xlink:to="loc_us-gaap_ClassOfStockDomain_019ef5c0-0bf9-7920-a1e6-5ac218ed2fea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019ef5c0-0bf9-779c-ac64-2268e35f10f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019ef5c0-0bf9-7920-a1e6-5ac218ed2fea" xlink:to="loc_us-gaap_ClassOfStockDomain_019ef5c0-0bf9-779c-ac64-2268e35f10f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019ef5c0-0bf9-7228-b17e-81257c849765" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019ef5c0-0bf9-779c-ac64-2268e35f10f0" xlink:to="loc_us-gaap_CommonStockMember_019ef5c0-0bf9-7228-b17e-81257c849765" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019ef5c0-0bf9-7c45-95bd-96ca0009f02a" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:to="loc_dei_LegalEntityAxis_019ef5c0-0bf9-7c45-95bd-96ca0009f02a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019ef5c0-0bf9-7c45-95bd-96ca0009f02a_default" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019ef5c0-0bf9-7c45-95bd-96ca0009f02a" xlink:to="loc_dei_EntityDomain_019ef5c0-0bf9-7c45-95bd-96ca0009f02a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019ef5c0-0bf9-7fff-995c-788cd22c7d51" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019ef5c0-0bf9-7c45-95bd-96ca0009f02a" xlink:to="loc_dei_EntityDomain_019ef5c0-0bf9-7fff-995c-788cd22c7d51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_GGPHoldingsLPMember_019ef5c0-0bf9-7d6a-93c6-8a8f321ce111" xlink:href="gbdc4-20250930.xsd#gbdc4_GGPHoldingsLPMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019ef5c0-0bf9-7fff-995c-788cd22c7d51" xlink:to="loc_gbdc4_GGPHoldingsLPMember_019ef5c0-0bf9-7d6a-93c6-8a8f321ce111" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019ef5c0-0bf9-7550-a300-a9513cae0876" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019ef5c0-0bf9-7c06-9fa0-1ae45ff323e9" xlink:to="loc_us-gaap_CreditFacilityAxis_019ef5c0-0bf9-7550-a300-a9513cae0876" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bf9-7550-a300-a9513cae0876_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019ef5c0-0bf9-7550-a300-a9513cae0876" xlink:to="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bf9-7550-a300-a9513cae0876_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bf9-7e6a-acd8-db15b1947454" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019ef5c0-0bf9-7550-a300-a9513cae0876" xlink:to="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bf9-7e6a-acd8-db15b1947454" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019ef5c0-0bf9-757d-8ef1-10fc8758b008" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bf9-7e6a-acd8-db15b1947454" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019ef5c0-0bf9-757d-8ef1-10fc8758b008" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ManagementAndServiceFeesNumberOfComponents_019ef5c0-0bf9-7bd8-b921-82bf01ffebab" xlink:href="gbdc4-20250930.xsd#gbdc4_ManagementAndServiceFeesNumberOfComponents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_ManagementAndServiceFeesNumberOfComponents_019ef5c0-0bf9-7bd8-b921-82bf01ffebab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ManagementAndServiceFeesBaseRate_019ef5c0-0bf9-7924-acb3-308a72aa75a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ManagementAndServiceFeesBaseRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_ManagementAndServiceFeesBaseRate_019ef5c0-0bf9-7924-acb3-308a72aa75a5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ManagementAndServiceFeesCashEquivalentsMaturityPeriod_019ef5c0-0bf9-72a4-bdff-05e2bb789d68" xlink:href="gbdc4-20250930.xsd#gbdc4_ManagementAndServiceFeesCashEquivalentsMaturityPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_ManagementAndServiceFeesCashEquivalentsMaturityPeriod_019ef5c0-0bf9-72a4-bdff-05e2bb789d68" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyLiquidityEventManagementAndServiceFeesBaseRateWaiverThresholdPercentageOfFairValueOfAverageAdjustedGrossAssets_019ef5c0-0bf9-7159-9d86-783d6821e5df" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyLiquidityEventManagementAndServiceFeesBaseRateWaiverThresholdPercentageOfFairValueOfAverageAdjustedGrossAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_InvestmentCompanyLiquidityEventManagementAndServiceFeesBaseRateWaiverThresholdPercentageOfFairValueOfAverageAdjustedGrossAssets_019ef5c0-0bf9-7159-9d86-783d6821e5df" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ManagementFeeExpense_019ef5c0-0bf9-7ea5-93f6-8803137148a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ManagementFeeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_ManagementFeeExpense_019ef5c0-0bf9-7ea5-93f6-8803137148a4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyManagementFeeContractualFeeWaived_019ef5c0-0bf9-744b-a282-faf75369a76f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyManagementFeeContractualFeeWaived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_InvestmentCompanyManagementFeeContractualFeeWaived_019ef5c0-0bf9-744b-a282-faf75369a76f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyNumberOfIncentiveFeeComponents_019ef5c0-0bf9-790e-9dc5-3c99819dc68a" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyNumberOfIncentiveFeeComponents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_InvestmentCompanyNumberOfIncentiveFeeComponents_019ef5c0-0bf9-790e-9dc5-3c99819dc68a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ManagementAndServiceFeesIncentiveRate_019ef5c0-0bf9-7147-9d36-f05f31f61954" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ManagementAndServiceFeesIncentiveRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_ManagementAndServiceFeesIncentiveRate_019ef5c0-0bf9-7147-9d36-f05f31f61954" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncentiveFeeExpenseIncomeBased_019ef5c0-0bf9-7a31-83a8-375564e5ab86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncentiveFeeExpenseIncomeBased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_IncentiveFeeExpenseIncomeBased_019ef5c0-0bf9-7a31-83a8-375564e5ab86" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyIncentiveFeeWaivedIncomeBased_019ef5c0-0bf9-7a73-805a-6af13f5ea87e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyIncentiveFeeWaivedIncomeBased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_InvestmentCompanyIncentiveFeeWaivedIncomeBased_019ef5c0-0bf9-7a73-805a-6af13f5ea87e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyLiquidityEventManagementAndServiceFeesIncentiveRateWaiverThresholdPercentageOfCapitalGainIncentiveFeeBase_019ef5c0-0bf9-718e-823b-ef8b64491d47" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyLiquidityEventManagementAndServiceFeesIncentiveRateWaiverThresholdPercentageOfCapitalGainIncentiveFeeBase"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_InvestmentCompanyLiquidityEventManagementAndServiceFeesIncentiveRateWaiverThresholdPercentageOfCapitalGainIncentiveFeeBase_019ef5c0-0bf9-718e-823b-ef8b64491d47" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInIncentiveFeePayableCapitalGainBased_019ef5c0-0bf9-733c-8407-247e7b0e6221" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInIncentiveFeePayableCapitalGainBased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_IncreaseDecreaseInIncentiveFeePayableCapitalGainBased_019ef5c0-0bf9-733c-8407-247e7b0e6221" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncentiveFeePayableCapitalGainBased_019ef5c0-0bf9-7cd2-bf43-5a946f54e32e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncentiveFeePayableCapitalGainBased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_IncentiveFeePayableCapitalGainBased_019ef5c0-0bf9-7cd2-bf43-5a946f54e32e" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PaymentForCapitalGainsIncentiveFee_019ef5c0-0bf9-7076-a50c-1d0f465f88ea" xlink:href="gbdc4-20250930.xsd#gbdc4_PaymentForCapitalGainsIncentiveFee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_PaymentForCapitalGainsIncentiveFee_019ef5c0-0bf9-7076-a50c-1d0f465f88ea" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ManagementAndServiceFeesIncentiveRateCapPercentageOfCumulativePreIncentiveFeeNetIncome_019ef5c0-0bf9-7b3f-b972-904218ae8c2c" xlink:href="gbdc4-20250930.xsd#gbdc4_ManagementAndServiceFeesIncentiveRateCapPercentageOfCumulativePreIncentiveFeeNetIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_ManagementAndServiceFeesIncentiveRateCapPercentageOfCumulativePreIncentiveFeeNetIncome_019ef5c0-0bf9-7b3f-b972-904218ae8c2c" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent_019ef5c0-0bf9-7b40-ac35-cece9716d53f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent_019ef5c0-0bf9-7b40-ac35-cece9716d53f" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrganizationalCostExpense_019ef5c0-0bf9-7115-8a62-c352e16bde38" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OrganizationalCostExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_OrganizationalCostExpense_019ef5c0-0bf9-7115-8a62-c352e16bde38" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyExcessExpenseReimbursable_019ef5c0-0bf9-78aa-a76a-f3091177de3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyExcessExpenseReimbursable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_InvestmentCompanyExcessExpenseReimbursable_019ef5c0-0bf9-78aa-a76a-f3091177de3c" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyReimbursableExcessExpenseAmountPaid_019ef5c0-0bf9-79de-9979-d89a814c17ca" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyReimbursableExcessExpenseAmountPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_InvestmentCompanyReimbursableExcessExpenseAmountPaid_019ef5c0-0bf9-79de-9979-d89a814c17ca" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019ef5c0-0bf9-7167-adcc-b5bd24834933" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019ef5c0-0bf9-7167-adcc-b5bd24834933" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanySubscribedCapital_019ef5c0-0bf9-7f98-99d4-cce8f70ade73" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanySubscribedCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_InvestmentCompanySubscribedCapital_019ef5c0-0bf9-7f98-99d4-cce8f70ade73" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_RelatedPartyTransactionShareTransferred_019ef5c0-0bf9-7dae-8572-b0a2e3b00cf2" xlink:href="gbdc4-20250930.xsd#gbdc4_RelatedPartyTransactionShareTransferred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_RelatedPartyTransactionShareTransferred_019ef5c0-0bf9-7dae-8572-b0a2e3b00cf2" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019ef5c0-0bfa-7a43-acf1-7d5d10110adc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_CommonStockSharesIssued_019ef5c0-0bfa-7a43-acf1-7d5d10110adc" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyContributedCapital_019ef5c0-0bfa-70fc-b7d0-2d9cdbdd80f1" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyContributedCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_InvestmentCompanyContributedCapital_019ef5c0-0bfa-70fc-b7d0-2d9cdbdd80f1" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CommonStockSharesIssuedDividendReinvestmentPlan_019ef5c0-0bfa-73a1-9686-0c6b25257dba" xlink:href="gbdc4-20250930.xsd#gbdc4_CommonStockSharesIssuedDividendReinvestmentPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_gbdc4_CommonStockSharesIssuedDividendReinvestmentPlan_019ef5c0-0bfa-73a1-9686-0c6b25257dba" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019ef5c0-0bfa-754f-95d6-ad96da717bf5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019ef5c0-0bf9-7675-bbd1-c44bc4c85eeb" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019ef5c0-0bfa-754f-95d6-ad96da717bf5" xlink:type="arc" order="25"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/InvestmentsScheduleofFairValueAssetsMeasuredonRecurringBasisDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#InvestmentsScheduleofFairValueAssetsMeasuredonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/InvestmentsScheduleofFairValueAssetsMeasuredonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-74aa-89b9-146e9d8fd1a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-7a24-b817-8b36c95f518f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-74aa-89b9-146e9d8fd1a7" xlink:to="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-7a24-b817-8b36c95f518f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bfa-726a-8a8a-80c7b49c062b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-7a24-b817-8b36c95f518f" xlink:to="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bfa-726a-8a8a-80c7b49c062b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-726a-8a8a-80c7b49c062b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bfa-726a-8a8a-80c7b49c062b" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-726a-8a8a-80c7b49c062b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-77d8-a34c-ef5e0c1a856f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeCategorizationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InvestmentTypeAxis_019ef5c0-0bfa-726a-8a8a-80c7b49c062b" xlink:to="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-77d8-a34c-ef5e0c1a856f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritiesSeniorSecuredLoanMember_019ef5c0-0bfa-7e80-82d7-dfab4ab96f03" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritiesSeniorSecuredLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-77d8-a34c-ef5e0c1a856f" xlink:to="loc_gbdc4_DebtSecuritiesSeniorSecuredLoanMember_019ef5c0-0bfa-7e80-82d7-dfab4ab96f03" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritiesOneStopLoansMember_019ef5c0-0bfa-7fb7-abe9-c27cde94ee65" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritiesOneStopLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-77d8-a34c-ef5e0c1a856f" xlink:to="loc_gbdc4_DebtSecuritiesOneStopLoansMember_019ef5c0-0bfa-7fb7-abe9-c27cde94ee65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritiesSecondLienMember_019ef5c0-0bfa-7bd6-a7cf-f874d784cd91" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritiesSecondLienMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-77d8-a34c-ef5e0c1a856f" xlink:to="loc_gbdc4_DebtSecuritiesSecondLienMember_019ef5c0-0bfa-7bd6-a7cf-f874d784cd91" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebtMember_019ef5c0-0bfa-7262-bdf5-edac530b9050" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-77d8-a34c-ef5e0c1a856f" xlink:to="loc_us-gaap_SubordinatedDebtMember_019ef5c0-0bfa-7262-bdf5-edac530b9050" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bfa-77fa-8e8d-c019a8d172eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentTypeCategorizationMember_019ef5c0-0bfa-77d8-a34c-ef5e0c1a856f" xlink:to="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bfa-77fa-8e8d-c019a8d172eb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue_019ef5c0-0bfa-7636-8479-652ce864e103" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-74aa-89b9-146e9d8fd1a7" xlink:to="loc_us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue_019ef5c0-0bfa-7636-8479-652ce864e103" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bfa-7fb7-a5aa-d7610d81234c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-74aa-89b9-146e9d8fd1a7" xlink:to="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bfa-7fb7-a5aa-d7610d81234c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-74f6-81bd-b283930f6a70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-74aa-89b9-146e9d8fd1a7" xlink:to="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-74f6-81bd-b283930f6a70" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/InvestmentsSchedulesofPortfolioCompositionbyGeographicRegionatAmortizationCostandFairValueDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#InvestmentsSchedulesofPortfolioCompositionbyGeographicRegionatAmortizationCostandFairValueDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/InvestmentsSchedulesofPortfolioCompositionbyGeographicRegionatAmortizationCostandFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-72ce-894b-fe33e42bb3ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-7c17-809c-1774e4ce6866" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-72ce-894b-fe33e42bb3ec" xlink:to="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-7c17-809c-1774e4ce6866" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019ef5c0-0bfa-7bee-90f9-b946bfc12468" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-7c17-809c-1774e4ce6866" xlink:to="loc_srt_StatementGeographicalAxis_019ef5c0-0bfa-7bee-90f9-b946bfc12468" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7bee-90f9-b946bfc12468_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019ef5c0-0bfa-7bee-90f9-b946bfc12468" xlink:to="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7bee-90f9-b946bfc12468_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019ef5c0-0bfa-7bee-90f9-b946bfc12468" xlink:to="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MidwestRegionMember_019ef5c0-0bfa-7751-9840-036c2a0a786a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MidwestRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_us-gaap_MidwestRegionMember_019ef5c0-0bfa-7751-9840-036c2a0a786a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WestRegionMember_019ef5c0-0bfa-7cbd-8182-1eac641f2751" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WestRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_us-gaap_WestRegionMember_019ef5c0-0bfa-7cbd-8182-1eac641f2751" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoutheastRegionMember_019ef5c0-0bfa-7c8d-9678-3cd4da5a2144" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoutheastRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_us-gaap_SoutheastRegionMember_019ef5c0-0bfa-7c8d-9678-3cd4da5a2144" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SouthwestRegionMember_019ef5c0-0bfa-7d1f-8eb6-9bcd8b2fdb75" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SouthwestRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_us-gaap_SouthwestRegionMember_019ef5c0-0bfa-7d1f-8eb6-9bcd8b2fdb75" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_USMidAtlanticMember_019ef5c0-0bfa-770b-819c-41cd7a0e6680" xlink:href="gbdc4-20250930.xsd#gbdc4_USMidAtlanticMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_gbdc4_USMidAtlanticMember_019ef5c0-0bfa-770b-819c-41cd7a0e6680" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NortheastRegionMember_019ef5c0-0bfa-780f-af3e-9c60d0a7beee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NortheastRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_us-gaap_NortheastRegionMember_019ef5c0-0bfa-780f-af3e-9c60d0a7beee" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_019ef5c0-0bfa-7c67-a88b-67373c931b1c" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_GB_019ef5c0-0bfa-7c67-a88b-67373c931b1c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_DE_019ef5c0-0bfa-742d-b65b-3e9e5a8eeb22" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_DE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_DE_019ef5c0-0bfa-742d-b65b-3e9e5a8eeb22" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_019ef5c0-0bfa-70b7-879b-387489feeacc" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_CA_019ef5c0-0bfa-70b7-879b-387489feeacc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_AU_019ef5c0-0bfa-7d42-b075-9fba63ce309b" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_AU"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_AU_019ef5c0-0bfa-7d42-b075-9fba63ce309b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_JE_019ef5c0-0bfa-79e8-85e6-12938158f96a" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_JE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_JE_019ef5c0-0bfa-79e8-85e6-12938158f96a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_FI_019ef5c0-0bfa-7a79-8723-e44f768ef224" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_FI"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_FI_019ef5c0-0bfa-7a79-8723-e44f768ef224" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_SE_019ef5c0-0bfa-7706-85a8-739dc032da45" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_SE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_SE_019ef5c0-0bfa-7706-85a8-739dc032da45" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_NL_019ef5c0-0bfa-7a1b-84c5-05d617c34f70" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_NL"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_NL_019ef5c0-0bfa-7a1b-84c5-05d617c34f70" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_FR_019ef5c0-0bfa-7ec2-a2be-873762c03975" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_FR"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_FR_019ef5c0-0bfa-7ec2-a2be-873762c03975" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_DK_019ef5c0-0bfa-7c06-974b-fc971e992fa8" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_DK"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_DK_019ef5c0-0bfa-7c06-974b-fc971e992fa8" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_LU_019ef5c0-0bfa-7aec-983f-b12a97343d6c" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_LU"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_LU_019ef5c0-0bfa-7aec-983f-b12a97343d6c" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IL_019ef5c0-0bfa-727e-a255-2b4871d38541" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_IL"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019ef5c0-0bfa-7a00-9ece-9ca3918001cc" xlink:to="loc_country_IL_019ef5c0-0bfa-727e-a255-2b4871d38541" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bfa-78bc-a4a9-57383081f2d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-7c17-809c-1774e4ce6866" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bfa-78bc-a4a9-57383081f2d6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-78bc-a4a9-57383081f2d6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bfa-78bc-a4a9-57383081f2d6" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-78bc-a4a9-57383081f2d6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7188-a294-92110b8d4fa8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bfa-78bc-a4a9-57383081f2d6" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7188-a294-92110b8d4fa8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentsAtAmortizedCostMember_019ef5c0-0bfa-7f41-a1f9-8c659dd2f330" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentsAtAmortizedCostMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7188-a294-92110b8d4fa8" xlink:to="loc_gbdc4_InvestmentsAtAmortizedCostMember_019ef5c0-0bfa-7f41-a1f9-8c659dd2f330" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentsAtFairValueMember_019ef5c0-0bfa-77a7-9617-0c97db0cd27c" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentsAtFairValueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7188-a294-92110b8d4fa8" xlink:to="loc_gbdc4_InvestmentsAtFairValueMember_019ef5c0-0bfa-77a7-9617-0c97db0cd27c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bfa-7615-ba5b-5d08672041c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-7c17-809c-1774e4ce6866" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bfa-7615-ba5b-5d08672041c9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-7615-ba5b-5d08672041c9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bfa-7615-ba5b-5d08672041c9" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-7615-ba5b-5d08672041c9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-7595-b647-284668c4649b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bfa-7615-ba5b-5d08672041c9" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-7595-b647-284668c4649b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeographicConcentrationRiskMember_019ef5c0-0bfa-71e6-a63e-98f58095c43e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeographicConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-7595-b647-284668c4649b" xlink:to="loc_us-gaap_GeographicConcentrationRiskMember_019ef5c0-0bfa-71e6-a63e-98f58095c43e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bfa-7137-97ef-223d322d91be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-72ce-894b-fe33e42bb3ec" xlink:to="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bfa-7137-97ef-223d322d91be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019ef5c0-0bfa-7574-97ac-12d45bd9d22a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-72ce-894b-fe33e42bb3ec" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019ef5c0-0bfa-7574-97ac-12d45bd9d22a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-7d81-b100-08966472e42a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-72ce-894b-fe33e42bb3ec" xlink:to="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-7d81-b100-08966472e42a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/InvestmentsScheduleofPortfolioCompositionbyIndustryatAmortizationCostandFairValueDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#InvestmentsScheduleofPortfolioCompositionbyIndustryatAmortizationCostandFairValueDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/InvestmentsScheduleofPortfolioCompositionbyIndustryatAmortizationCostandFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-70a3-961a-3765b4782756" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-717e-8512-88fe6a3fdced" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-70a3-961a-3765b4782756" xlink:to="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-717e-8512-88fe6a3fdced" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bfa-7ac7-ab05-b998f6f48a6f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesByIndustryAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-717e-8512-88fe6a3fdced" xlink:to="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bfa-7ac7-ab05-b998f6f48a6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-7ac7-ab05-b998f6f48a6f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bfa-7ac7-ab05-b998f6f48a6f" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-7ac7-ab05-b998f6f48a6f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesIndustryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EquitySecuritiesByIndustryAxis_019ef5c0-0bfa-7ac7-ab05-b998f6f48a6f" xlink:to="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AerospaceAndDefenseSectorMember_019ef5c0-0bfa-743b-92dd-032ec5912706" xlink:href="gbdc4-20250930.xsd#gbdc4_AerospaceAndDefenseSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_AerospaceAndDefenseSectorMember_019ef5c0-0bfa-743b-92dd-032ec5912706" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AirFreightAndLogisticsMember_019ef5c0-0bfa-71ea-97fe-8cdb230f3422" xlink:href="gbdc4-20250930.xsd#gbdc4_AirFreightAndLogisticsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_AirFreightAndLogisticsMember_019ef5c0-0bfa-71ea-97fe-8cdb230f3422" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AutoComponentsSectorMember_019ef5c0-0bfa-76b7-be0b-265d36c23462" xlink:href="gbdc4-20250930.xsd#gbdc4_AutoComponentsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_AutoComponentsSectorMember_019ef5c0-0bfa-76b7-be0b-265d36c23462" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AutomobilesSectorMember_019ef5c0-0bfa-7440-b0f4-5fcb6d7acadd" xlink:href="gbdc4-20250930.xsd#gbdc4_AutomobilesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_AutomobilesSectorMember_019ef5c0-0bfa-7440-b0f4-5fcb6d7acadd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BanksSectorMember_019ef5c0-0bfa-73cc-876c-0a154b97e852" xlink:href="gbdc4-20250930.xsd#gbdc4_BanksSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_BanksSectorMember_019ef5c0-0bfa-73cc-876c-0a154b97e852" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BeveragesSectorMember_019ef5c0-0bfa-7993-937e-87ddd952b27b" xlink:href="gbdc4-20250930.xsd#gbdc4_BeveragesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_BeveragesSectorMember_019ef5c0-0bfa-7993-937e-87ddd952b27b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BuildingProductsSectorMember_019ef5c0-0bfa-7582-9cc1-e9a1f9124f4f" xlink:href="gbdc4-20250930.xsd#gbdc4_BuildingProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_BuildingProductsSectorMember_019ef5c0-0bfa-7582-9cc1-e9a1f9124f4f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CapitalMarketsSectorMember_019ef5c0-0bfa-70d7-b29c-92a0d989ea87" xlink:href="gbdc4-20250930.xsd#gbdc4_CapitalMarketsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_CapitalMarketsSectorMember_019ef5c0-0bfa-70d7-b29c-92a0d989ea87" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChemicalsSectorMember_019ef5c0-0bfa-7fe4-9df6-5994f6a26d62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChemicalsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_us-gaap_ChemicalsSectorMember_019ef5c0-0bfa-7fe4-9df6-5994f6a26d62" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CommercialServicesAndSuppliesSectorMember_019ef5c0-0bfa-7b57-aac7-bea5e939d7cc" xlink:href="gbdc4-20250930.xsd#gbdc4_CommercialServicesAndSuppliesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_CommercialServicesAndSuppliesSectorMember_019ef5c0-0bfa-7b57-aac7-bea5e939d7cc" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ConstructionAndEngineeringSectorMember_019ef5c0-0bfa-79a8-b6ae-ce7140385929" xlink:href="gbdc4-20250930.xsd#gbdc4_ConstructionAndEngineeringSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_ConstructionAndEngineeringSectorMember_019ef5c0-0bfa-79a8-b6ae-ce7140385929" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContainerAndPackagingSectorMember_019ef5c0-0bfa-779b-90c7-c3a9deef16c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContainerAndPackagingSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_us-gaap_ContainerAndPackagingSectorMember_019ef5c0-0bfa-779b-90c7-c3a9deef16c4" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DiversifiedConsumerServicesSectorMember_019ef5c0-0bfa-733b-8329-6ec722a55633" xlink:href="gbdc4-20250930.xsd#gbdc4_DiversifiedConsumerServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_DiversifiedConsumerServicesSectorMember_019ef5c0-0bfa-733b-8329-6ec722a55633" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DiversifiedFinancialServicesSectorMember_019ef5c0-0bfa-7e71-97ac-b5fe6107ed72" xlink:href="gbdc4-20250930.xsd#gbdc4_DiversifiedFinancialServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_DiversifiedFinancialServicesSectorMember_019ef5c0-0bfa-7e71-97ac-b5fe6107ed72" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectricUtilitiesSectorMember_019ef5c0-0bfa-73e9-a285-0f3676b21532" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectricUtilitiesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_ElectricUtilitiesSectorMember_019ef5c0-0bfa-73e9-a285-0f3676b21532" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectricalEquipmentSectorMember_019ef5c0-0bfa-7d82-8c48-9750f8cb0ded" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectricalEquipmentSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_ElectricalEquipmentSectorMember_019ef5c0-0bfa-7d82-8c48-9750f8cb0ded" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember_019ef5c0-0bfa-7ee4-b487-290bc4cdba6c" xlink:href="gbdc4-20250930.xsd#gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_ElectronicEquipmentInstrumentsAndComponentsSectorMember_019ef5c0-0bfa-7ee4-b487-290bc4cdba6c" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_FoodProductsSectorMember_019ef5c0-0bfa-7a0c-8fd4-8181fe6bbd34" xlink:href="gbdc4-20250930.xsd#gbdc4_FoodProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_FoodProductsSectorMember_019ef5c0-0bfa-7a0c-8fd4-8181fe6bbd34" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareEquipmentAndSuppliesSectorMember_019ef5c0-0bfa-7520-8711-a8dbb2ff4da9" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareEquipmentAndSuppliesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_HealthcareEquipmentAndSuppliesSectorMember_019ef5c0-0bfa-7520-8711-a8dbb2ff4da9" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareProvidersAndServicesSectorMember_019ef5c0-0bfa-730a-a36b-fb4c4851ce1b" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareProvidersAndServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_HealthcareProvidersAndServicesSectorMember_019ef5c0-0bfa-730a-a36b-fb4c4851ce1b" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HealthcareTechnologySectorMember_019ef5c0-0bfa-7e44-b9e2-712d838b704c" xlink:href="gbdc4-20250930.xsd#gbdc4_HealthcareTechnologySectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_HealthcareTechnologySectorMember_019ef5c0-0bfa-7e44-b9e2-712d838b704c" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HotelsRestaurantsAndLeisureSectorMember_019ef5c0-0bfa-79eb-8377-faf57053eabe" xlink:href="gbdc4-20250930.xsd#gbdc4_HotelsRestaurantsAndLeisureSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_HotelsRestaurantsAndLeisureSectorMember_019ef5c0-0bfa-79eb-8377-faf57053eabe" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_HouseholdProductsSectorMember_019ef5c0-0bfa-7121-9cee-6c96db5732af" xlink:href="gbdc4-20250930.xsd#gbdc4_HouseholdProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_HouseholdProductsSectorMember_019ef5c0-0bfa-7121-9cee-6c96db5732af" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_IndustrialConglomeratesSectorMember_019ef5c0-0bfa-7cff-801a-88f69e258270" xlink:href="gbdc4-20250930.xsd#gbdc4_IndustrialConglomeratesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_IndustrialConglomeratesSectorMember_019ef5c0-0bfa-7cff-801a-88f69e258270" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InsuranceSectorMember_019ef5c0-0bfa-747f-aa0b-183ddd367292" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InsuranceSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_us-gaap_InsuranceSectorMember_019ef5c0-0bfa-747f-aa0b-183ddd367292" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ITServicesSectorMember_019ef5c0-0bfa-746d-8ddd-4be2e7bc5473" xlink:href="gbdc4-20250930.xsd#gbdc4_ITServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_ITServicesSectorMember_019ef5c0-0bfa-746d-8ddd-4be2e7bc5473" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LeisureProductsSectorMember_019ef5c0-0bfa-76cd-b252-78dcbb25bb21" xlink:href="gbdc4-20250930.xsd#gbdc4_LeisureProductsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_LeisureProductsSectorMember_019ef5c0-0bfa-76cd-b252-78dcbb25bb21" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LifeSciencesToolsAndServicesSectorMember_019ef5c0-0bfa-7c06-b069-f79be90c4ddf" xlink:href="gbdc4-20250930.xsd#gbdc4_LifeSciencesToolsAndServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_LifeSciencesToolsAndServicesSectorMember_019ef5c0-0bfa-7c06-b069-f79be90c4ddf" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MachinerySectorMember_019ef5c0-0bfa-7c1b-bd82-ce7b33d14d48" xlink:href="gbdc4-20250930.xsd#gbdc4_MachinerySectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_MachinerySectorMember_019ef5c0-0bfa-7c1b-bd82-ce7b33d14d48" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MediaSectorMember_019ef5c0-0bfa-7126-8fcd-2c8f8b363deb" xlink:href="gbdc4-20250930.xsd#gbdc4_MediaSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_MediaSectorMember_019ef5c0-0bfa-7126-8fcd-2c8f8b363deb" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_OilGasAndConsumableFuelsSectorMember_019ef5c0-0bfa-7a6c-ba0d-50359f1f31d7" xlink:href="gbdc4-20250930.xsd#gbdc4_OilGasAndConsumableFuelsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_OilGasAndConsumableFuelsSectorMember_019ef5c0-0bfa-7a6c-ba0d-50359f1f31d7" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PharmaceuticalsSectorMember_019ef5c0-0bfa-761d-b724-5a26c4ac6725" xlink:href="gbdc4-20250930.xsd#gbdc4_PharmaceuticalsSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_PharmaceuticalsSectorMember_019ef5c0-0bfa-761d-b724-5a26c4ac6725" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ProfessionalServicesSectorMember_019ef5c0-0bfa-7901-8a19-8ce9c6d00556" xlink:href="gbdc4-20250930.xsd#gbdc4_ProfessionalServicesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_ProfessionalServicesSectorMember_019ef5c0-0bfa-7901-8a19-8ce9c6d00556" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_RoadRailSectorMember_019ef5c0-0bfa-7a51-92b6-1985e7147bd7" xlink:href="gbdc4-20250930.xsd#gbdc4_RoadRailSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_RoadRailSectorMember_019ef5c0-0bfa-7a51-92b6-1985e7147bd7" xlink:type="arc" order="33"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SoftwareSectorMember_019ef5c0-0bfa-7ff5-bb3d-a6b8a87d0cb2" xlink:href="gbdc4-20250930.xsd#gbdc4_SoftwareSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_SoftwareSectorMember_019ef5c0-0bfa-7ff5-bb3d-a6b8a87d0cb2" xlink:type="arc" order="34"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SpecialtyRetailSectorMember_019ef5c0-0bfa-7158-843d-0bd26c58fa46" xlink:href="gbdc4-20250930.xsd#gbdc4_SpecialtyRetailSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_SpecialtyRetailSectorMember_019ef5c0-0bfa-7158-843d-0bd26c58fa46" xlink:type="arc" order="35"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_TransportationInfrastructureSectorMember_019ef5c0-0bfa-7e2f-93db-3f225a8097d4" xlink:href="gbdc4-20250930.xsd#gbdc4_TransportationInfrastructureSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_TransportationInfrastructureSectorMember_019ef5c0-0bfa-7e2f-93db-3f225a8097d4" xlink:type="arc" order="36"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_WaterUtilitiesSectorMember_019ef5c0-0bfa-7afd-ae2f-376b0da28c1b" xlink:href="gbdc4-20250930.xsd#gbdc4_WaterUtilitiesSectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquitySecuritiesIndustryMember_019ef5c0-0bfa-705f-86f6-c6053ea82aa0" xlink:to="loc_gbdc4_WaterUtilitiesSectorMember_019ef5c0-0bfa-7afd-ae2f-376b0da28c1b" xlink:type="arc" order="37"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bfa-7830-b492-5e8d5878acfd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-717e-8512-88fe6a3fdced" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bfa-7830-b492-5e8d5878acfd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7830-b492-5e8d5878acfd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bfa-7830-b492-5e8d5878acfd" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7830-b492-5e8d5878acfd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7970-9a06-b539b0ee695d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019ef5c0-0bfa-7830-b492-5e8d5878acfd" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7970-9a06-b539b0ee695d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentsAtAmortizedCostMember_019ef5c0-0bfa-78d1-a970-eb0701977b33" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentsAtAmortizedCostMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7970-9a06-b539b0ee695d" xlink:to="loc_gbdc4_InvestmentsAtAmortizedCostMember_019ef5c0-0bfa-78d1-a970-eb0701977b33" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentsAtFairValueMember_019ef5c0-0bfa-7fde-876d-83ff569000d5" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentsAtFairValueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019ef5c0-0bfa-7970-9a06-b539b0ee695d" xlink:to="loc_gbdc4_InvestmentsAtFairValueMember_019ef5c0-0bfa-7fde-876d-83ff569000d5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bfa-744b-9bae-2c4ab663f56e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfInvestmentsTable_019ef5c0-0bfa-717e-8512-88fe6a3fdced" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bfa-744b-9bae-2c4ab663f56e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-744b-9bae-2c4ab663f56e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bfa-744b-9bae-2c4ab663f56e" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-744b-9bae-2c4ab663f56e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-70c1-8a6e-46cc0a7f8b39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019ef5c0-0bfa-744b-9bae-2c4ab663f56e" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-70c1-8a6e-46cc0a7f8b39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_IndustryConcentrationRiskMember_019ef5c0-0bfa-737b-9247-447236e5f201" xlink:href="gbdc4-20250930.xsd#gbdc4_IndustryConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019ef5c0-0bfa-70c1-8a6e-46cc0a7f8b39" xlink:to="loc_gbdc4_IndustryConcentrationRiskMember_019ef5c0-0bfa-737b-9247-447236e5f201" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bfa-7999-9606-33f5ee5d466d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-70a3-961a-3765b4782756" xlink:to="loc_us-gaap_InvestmentOwnedAtCost_019ef5c0-0bfa-7999-9606-33f5ee5d466d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019ef5c0-0bfa-7b81-8d86-c0988be09fee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-70a3-961a-3765b4782756" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019ef5c0-0bfa-7b81-8d86-c0988be09fee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-78a6-a86d-3532f4b57803" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfInvestmentsLineItems_019ef5c0-0bfa-70a3-961a-3765b4782756" xlink:to="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-78a6-a86d-3532f4b57803" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/DerivativesScheduleofOpenForwardForeignCurrencyContractDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#DerivativesScheduleofOpenForwardForeignCurrencyContractDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/DerivativesScheduleofOpenForwardForeignCurrencyContractDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7309-bcc0-d491b42e6c08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OpenForwardForeignCurrencyContractLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-74bd-97e9-b2ce91db67f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OpenForwardForeignCurrencyContractTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7309-bcc0-d491b42e6c08" xlink:to="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-74bd-97e9-b2ce91db67f9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019ef5c0-0bfa-7390-aeab-e5968d35c4c1" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-74bd-97e9-b2ce91db67f9" xlink:to="loc_srt_CounterpartyNameAxis_019ef5c0-0bfa-7390-aeab-e5968d35c4c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-7390-aeab-e5968d35c4c1_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019ef5c0-0bfa-7390-aeab-e5968d35c4c1" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-7390-aeab-e5968d35c4c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-7d84-b8c9-d3647ebc0aba" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019ef5c0-0bfa-7390-aeab-e5968d35c4c1" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-7d84-b8c9-d3647ebc0aba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SMBCCapitalMarketsInc.Member_019ef5c0-0bfa-7f3b-a22a-dbe0b1da26df" xlink:href="gbdc4-20250930.xsd#gbdc4_SMBCCapitalMarketsInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-7d84-b8c9-d3647ebc0aba" xlink:to="loc_gbdc4_SMBCCapitalMarketsInc.Member_019ef5c0-0bfa-7f3b-a22a-dbe0b1da26df" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityNotionalAmount_019ef5c0-0bfa-7120-8e38-c1069b89275a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilityNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7309-bcc0-d491b42e6c08" xlink:to="loc_us-gaap_DerivativeLiabilityNotionalAmount_019ef5c0-0bfa-7120-8e38-c1069b89275a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetNotionalAmount_019ef5c0-0bfa-7bcd-b73f-f507d5a638c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7309-bcc0-d491b42e6c08" xlink:to="loc_us-gaap_DerivativeAssetNotionalAmount_019ef5c0-0bfa-7bcd-b73f-f507d5a638c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetFairValueGrossAssetIncludingNotSubjectToMasterNettingArrangement_019ef5c0-0bfa-7aee-8ee7-42bed7dcd586" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetFairValueGrossAssetIncludingNotSubjectToMasterNettingArrangement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7309-bcc0-d491b42e6c08" xlink:to="loc_us-gaap_DerivativeAssetFairValueGrossAssetIncludingNotSubjectToMasterNettingArrangement_019ef5c0-0bfa-7aee-8ee7-42bed7dcd586" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAssetFairValueGrossLiability_019ef5c0-0bfa-7e13-94ea-17f39bd61cd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeAssetFairValueGrossLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7309-bcc0-d491b42e6c08" xlink:to="loc_us-gaap_DerivativeAssetFairValueGrossLiability_019ef5c0-0bfa-7e13-94ea-17f39bd61cd5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019ef5c0-0bfa-7d7c-8581-15f5a063e980" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7309-bcc0-d491b42e6c08" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019ef5c0-0bfa-7d7c-8581-15f5a063e980" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019ef5c0-0bfa-7995-b1da-4ae2eb9620eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7309-bcc0-d491b42e6c08" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019ef5c0-0bfa-7995-b1da-4ae2eb9620eb" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/DerivativesScheduleofRealizedandUnrealizedGainLossonForwardCurrencyContractDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#DerivativesScheduleofRealizedandUnrealizedGainLossonForwardCurrencyContractDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/DerivativesScheduleofRealizedandUnrealizedGainLossonForwardCurrencyContractDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7ddc-b4ce-f235d710c714" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OpenForwardForeignCurrencyContractLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-742b-be78-47567d03f875" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OpenForwardForeignCurrencyContractTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7ddc-b4ce-f235d710c714" xlink:to="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-742b-be78-47567d03f875" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019ef5c0-0bfa-70da-b144-429772ba6255" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-742b-be78-47567d03f875" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019ef5c0-0bfa-70da-b144-429772ba6255" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-70da-b144-429772ba6255_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ef5c0-0bfa-70da-b144-429772ba6255" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-70da-b144-429772ba6255_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-785c-ba39-da3e7ab4e00c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ef5c0-0bfa-70da-b144-429772ba6255" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-785c-ba39-da3e7ab4e00c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeForwardMember_019ef5c0-0bfa-779e-bb2c-6d48bb9fbae3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignExchangeForwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-785c-ba39-da3e7ab4e00c" xlink:to="loc_us-gaap_ForeignExchangeForwardMember_019ef5c0-0bfa-779e-bb2c-6d48bb9fbae3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfDerivatives_019ef5c0-0bfa-721e-a797-dd873786735d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnSaleOfDerivatives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7ddc-b4ce-f235d710c714" xlink:to="loc_us-gaap_GainLossOnSaleOfDerivatives_019ef5c0-0bfa-721e-a797-dd873786735d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrealizedGainLossOnDerivatives_019ef5c0-0bfa-73ad-bd8c-96cd9ddb609b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrealizedGainLossOnDerivatives"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7ddc-b4ce-f235d710c714" xlink:to="loc_us-gaap_UnrealizedGainLossOnDerivatives_019ef5c0-0bfa-73ad-bd8c-96cd9ddb609b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/DerivativesScheduleofAverageOutstandingDailyVolumeforForwardCurrencyContractDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#DerivativesScheduleofAverageOutstandingDailyVolumeforForwardCurrencyContractDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/DerivativesScheduleofAverageOutstandingDailyVolumeforForwardCurrencyContractDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7134-85df-2f273f0ced26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OpenForwardForeignCurrencyContractLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-7074-923e-580ad3d5a601" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OpenForwardForeignCurrencyContractTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7134-85df-2f273f0ced26" xlink:to="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-7074-923e-580ad3d5a601" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_019ef5c0-0bfa-7d01-bc34-9d822fed0882" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-7074-923e-580ad3d5a601" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_019ef5c0-0bfa-7d01-bc34-9d822fed0882" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-7d01-bc34-9d822fed0882_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ef5c0-0bfa-7d01-bc34-9d822fed0882" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-7d01-bc34-9d822fed0882_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-7fe9-a808-194f218a57b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_019ef5c0-0bfa-7d01-bc34-9d822fed0882" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-7fe9-a808-194f218a57b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignExchangeForwardMember_019ef5c0-0bfa-707f-9cf3-0b005731a4f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignExchangeForwardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_019ef5c0-0bfa-7fe9-a808-194f218a57b0" xlink:to="loc_us-gaap_ForeignExchangeForwardMember_019ef5c0-0bfa-707f-9cf3-0b005731a4f2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DerivativeAverageNotionalAmount_019ef5c0-0bfa-7e57-8168-f3cdda244e6b" xlink:href="gbdc4-20250930.xsd#gbdc4_DerivativeAverageNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7134-85df-2f273f0ced26" xlink:to="loc_gbdc4_DerivativeAverageNotionalAmount_019ef5c0-0bfa-7e57-8168-f3cdda244e6b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/DerivativesScheduleofForwardCurrencyContractFairValueDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#DerivativesScheduleofForwardCurrencyContractFairValueDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/DerivativesScheduleofForwardCurrencyContractFairValueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7b2d-8cb6-7d7550c8427e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OpenForwardForeignCurrencyContractLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-7ac4-b372-e4264b561d0b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OpenForwardForeignCurrencyContractTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7b2d-8cb6-7d7550c8427e" xlink:to="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-7ac4-b372-e4264b561d0b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019ef5c0-0bfa-7ae4-8084-4aba3fa6cb91" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractTable_019ef5c0-0bfa-7ac4-b372-e4264b561d0b" xlink:to="loc_srt_CounterpartyNameAxis_019ef5c0-0bfa-7ae4-8084-4aba3fa6cb91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-7ae4-8084-4aba3fa6cb91_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019ef5c0-0bfa-7ae4-8084-4aba3fa6cb91" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-7ae4-8084-4aba3fa6cb91_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-706e-a042-9718aa8e441c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019ef5c0-0bfa-7ae4-8084-4aba3fa6cb91" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-706e-a042-9718aa8e441c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SMBCCapitalMarketsInc.Member_019ef5c0-0bfa-7a20-9088-0203aba168bb" xlink:href="gbdc4-20250930.xsd#gbdc4_SMBCCapitalMarketsInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019ef5c0-0bfa-706e-a042-9718aa8e441c" xlink:to="loc_gbdc4_SMBCCapitalMarketsInc.Member_019ef5c0-0bfa-7a20-9088-0203aba168bb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OffsettingDerivativeAssetsAbstract_019ef5c0-0bfa-7b34-8bae-a5f459187aa1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OffsettingDerivativeAssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OpenForwardForeignCurrencyContractLineItems_019ef5c0-0bfa-7b2d-8cb6-7d7550c8427e" xlink:to="loc_us-gaap_OffsettingDerivativeAssetsAbstract_019ef5c0-0bfa-7b34-8bae-a5f459187aa1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019ef5c0-0bfa-7a52-8569-b050b44576b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OffsettingDerivativeAssetsAbstract_019ef5c0-0bfa-7b34-8bae-a5f459187aa1" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeAsset_019ef5c0-0bfa-7a52-8569-b050b44576b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019ef5c0-0bfa-750e-affc-f44ed276cdbb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OffsettingDerivativeAssetsAbstract_019ef5c0-0bfa-7b34-8bae-a5f459187aa1" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiability_019ef5c0-0bfa-750e-affc-f44ed276cdbb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilities_019ef5c0-0bfa-7ec6-aaeb-d38c02a72894" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OffsettingDerivativeAssetsAbstract_019ef5c0-0bfa-7b34-8bae-a5f459187aa1" xlink:to="loc_us-gaap_DerivativeLiabilities_019ef5c0-0bfa-7ec6-aaeb-d38c02a72894" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeCollateralRightToReclaimCash_019ef5c0-0bfa-7556-b208-2a1f025bec82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeCollateralRightToReclaimCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OffsettingDerivativeAssetsAbstract_019ef5c0-0bfa-7b34-8bae-a5f459187aa1" xlink:to="loc_us-gaap_DerivativeCollateralRightToReclaimCash_019ef5c0-0bfa-7556-b208-2a1f025bec82" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral_019ef5c0-0bfa-76e9-9440-d04301d5ca29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OffsettingDerivativeAssetsAbstract_019ef5c0-0bfa-7b34-8bae-a5f459187aa1" xlink:to="loc_us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral_019ef5c0-0bfa-76e9-9440-d04301d5ca29" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/FairValueMeasurementsScheduleofFairValueAssetsMeasuredonRecurringBasisDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#FairValueMeasurementsScheduleofFairValueAssetsMeasuredonRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/FairValueMeasurementsScheduleofFairValueAssetsMeasuredonRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7484-a201-f7318d000664" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-7189-b017-919065e148ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7484-a201-f7318d000664" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-7189-b017-919065e148ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019ef5c0-0bfa-729b-af09-29f1776e2a50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-7189-b017-919065e148ac" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019ef5c0-0bfa-729b-af09-29f1776e2a50" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-729b-af09-29f1776e2a50_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019ef5c0-0bfa-729b-af09-29f1776e2a50" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-729b-af09-29f1776e2a50_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-7863-856b-624f40cc140c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019ef5c0-0bfa-729b-af09-29f1776e2a50" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-7863-856b-624f40cc140c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesMember_019ef5c0-0bfa-72c5-9d69-b8fd515bbf7b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-7863-856b-624f40cc140c" xlink:to="loc_us-gaap_DebtSecuritiesMember_019ef5c0-0bfa-72c5-9d69-b8fd515bbf7b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bfa-7b90-8cf5-fabb5e39a3d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-7863-856b-624f40cc140c" xlink:to="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bfa-7b90-8cf5-fabb5e39a3d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-7677-b148-cc3beedf237a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-7189-b017-919065e148ac" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-7677-b148-cc3beedf237a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7677-b148-cc3beedf237a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-7677-b148-cc3beedf237a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7677-b148-cc3beedf237a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7fb5-8a63-456e8e21c3f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-7677-b148-cc3beedf237a" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7fb5-8a63-456e8e21c3f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019ef5c0-0bfa-708f-8dc0-c666b1bb71a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7fb5-8a63-456e8e21c3f9" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019ef5c0-0bfa-708f-8dc0-c666b1bb71a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019ef5c0-0bfa-7f84-96d5-4dc99b45febc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7fb5-8a63-456e8e21c3f9" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019ef5c0-0bfa-7f84-96d5-4dc99b45febc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019ef5c0-0bfa-7833-b814-4d20d5fa76ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7fb5-8a63-456e8e21c3f9" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019ef5c0-0bfa-7833-b814-4d20d5fa76ab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-73fb-afb1-632c64b5f375" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7484-a201-f7318d000664" xlink:to="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-73fb-afb1-632c64b5f375" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019ef5c0-0bfa-7166-915b-2ddfa901183c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7484-a201-f7318d000664" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019ef5c0-0bfa-7166-915b-2ddfa901183c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019ef5c0-0bfa-7a18-82b3-41c39e9e1e15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7484-a201-f7318d000664" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019ef5c0-0bfa-7a18-82b3-41c39e9e1e15" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_019ef5c0-0bfa-7827-86a8-023dca3356a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7484-a201-f7318d000664" xlink:to="loc_us-gaap_ForeignCurrencyContractsLiabilityFairValueDisclosure_019ef5c0-0bfa-7827-86a8-023dca3356a9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_019ef5c0-0bfa-700a-9a11-05611355da00" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7484-a201-f7318d000664" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_019ef5c0-0bfa-700a-9a11-05611355da00" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#FairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7ad3-9438-b2eed803975c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-756b-857b-573178ebe6ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7ad3-9438-b2eed803975c" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-756b-857b-573178ebe6ee" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-7576-a2f2-8876d1e2b198" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-756b-857b-573178ebe6ee" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-7576-a2f2-8876d1e2b198" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7576-a2f2-8876d1e2b198_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-7576-a2f2-8876d1e2b198" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7576-a2f2-8876d1e2b198_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7705-a054-b9581eefdac1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-7576-a2f2-8876d1e2b198" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7705-a054-b9581eefdac1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019ef5c0-0bfa-7929-9313-7752d0a324b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7705-a054-b9581eefdac1" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019ef5c0-0bfa-7929-9313-7752d0a324b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax_019ef5c0-0bfa-7526-a953-e831540896dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7ad3-9438-b2eed803975c" xlink:to="loc_us-gaap_UnrealizedGainLossInvestmentDerivativeAndForeignCurrencyTransactionPriceChangeOperatingBeforeTax_019ef5c0-0bfa-7526-a953-e831540896dd" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/FairValueMeasurementsScheduleofFairValueAssetsMeasuredonRecurringBasisUnobservableInputReconciliationDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#FairValueMeasurementsScheduleofFairValueAssetsMeasuredonRecurringBasisUnobservableInputReconciliationDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/FairValueMeasurementsScheduleofFairValueAssetsMeasuredonRecurringBasisUnobservableInputReconciliationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-724e-ba63-75121b680728" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-7172-9698-44d3b0494b83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-724e-ba63-75121b680728" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-7172-9698-44d3b0494b83" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_019ef5c0-0bfa-744e-8d7e-eb731d7c3226" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-7172-9698-44d3b0494b83" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_019ef5c0-0bfa-744e-8d7e-eb731d7c3226" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-744e-8d7e-eb731d7c3226_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019ef5c0-0bfa-744e-8d7e-eb731d7c3226" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-744e-8d7e-eb731d7c3226_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-791b-9178-65d453295748" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_019ef5c0-0bfa-744e-8d7e-eb731d7c3226" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-791b-9178-65d453295748" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesMember_019ef5c0-0bfa-750f-9a7a-53cb592a25d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-791b-9178-65d453295748" xlink:to="loc_us-gaap_DebtSecuritiesMember_019ef5c0-0bfa-750f-9a7a-53cb592a25d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bfa-7b32-8192-93e293d27580" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_019ef5c0-0bfa-791b-9178-65d453295748" xlink:to="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bfa-7b32-8192-93e293d27580" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_019ef5c0-0bfa-7f28-a9af-6c58c8091156" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-7172-9698-44d3b0494b83" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_019ef5c0-0bfa-7f28-a9af-6c58c8091156" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7f28-a9af-6c58c8091156_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019ef5c0-0bfa-7f28-a9af-6c58c8091156" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7f28-a9af-6c58c8091156_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7485-9f0e-adc62038a8be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019ef5c0-0bfa-7f28-a9af-6c58c8091156" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7485-9f0e-adc62038a8be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtAndEquitySecuritiesUnrealizedGainLossMember_019ef5c0-0bfa-730f-823d-f84851e7b9b7" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtAndEquitySecuritiesUnrealizedGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7485-9f0e-adc62038a8be" xlink:to="loc_gbdc4_DebtAndEquitySecuritiesUnrealizedGainLossMember_019ef5c0-0bfa-730f-823d-f84851e7b9b7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_UnrealizedGainLossForeignCurrencyTransactionBeforeTaxMember_019ef5c0-0bfa-7391-b376-24016e5fd8d7" xlink:href="gbdc4-20250930.xsd#gbdc4_UnrealizedGainLossForeignCurrencyTransactionBeforeTaxMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7485-9f0e-adc62038a8be" xlink:to="loc_gbdc4_UnrealizedGainLossForeignCurrencyTransactionBeforeTaxMember_019ef5c0-0bfa-7391-b376-24016e5fd8d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtAndEquitySecuritiesRealizedGainLossMember_019ef5c0-0bfa-7e12-9003-b3d96d89bdd7" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtAndEquitySecuritiesRealizedGainLossMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7485-9f0e-adc62038a8be" xlink:to="loc_gbdc4_DebtAndEquitySecuritiesRealizedGainLossMember_019ef5c0-0bfa-7e12-9003-b3d96d89bdd7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_RealizedGainLossForeignCurrencyTransactionBeforeTaxMember_019ef5c0-0bfa-767e-bc20-725794c75a0b" xlink:href="gbdc4-20250930.xsd#gbdc4_RealizedGainLossForeignCurrencyTransactionBeforeTaxMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7485-9f0e-adc62038a8be" xlink:to="loc_gbdc4_RealizedGainLossForeignCurrencyTransactionBeforeTaxMember_019ef5c0-0bfa-767e-bc20-725794c75a0b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_GrossInvestmentIncomeOperatingMember_019ef5c0-0bfa-79ea-9774-973f83d43b16" xlink:href="gbdc4-20250930.xsd#gbdc4_GrossInvestmentIncomeOperatingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019ef5c0-0bfa-7485-9f0e-adc62038a8be" xlink:to="loc_gbdc4_GrossInvestmentIncomeOperatingMember_019ef5c0-0bfa-79ea-9774-973f83d43b16" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-724e-ba63-75121b680728" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019ef5c0-0bfa-74aa-909f-5cf39d0a2268" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019ef5c0-0bfa-74aa-909f-5cf39d0a2268" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019ef5c0-0bfa-7ff8-a0c6-ef3b860b857f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1_019ef5c0-0bfa-7ff8-a0c6-ef3b860b857f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssuancesSettlementsOfUndrawnPortionOfRevolvingLoans_019ef5c0-0bfa-7146-8b22-352326a46d6c" xlink:href="gbdc4-20250930.xsd#gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssuancesSettlementsOfUndrawnPortionOfRevolvingLoans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:to="loc_gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssuancesSettlementsOfUndrawnPortionOfRevolvingLoans_019ef5c0-0bfa-7146-8b22-352326a46d6c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues_019ef5c0-0bfa-78fa-a240-c0d596da4bf3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues_019ef5c0-0bfa-78fa-a240-c0d596da4bf3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPaidInKindInterest_019ef5c0-0bfa-77d2-8978-f849d85b9c05" xlink:href="gbdc4-20250930.xsd#gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPaidInKindInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:to="loc_gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPaidInKindInterest_019ef5c0-0bfa-77d2-8978-f849d85b9c05" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetProceedsFromNonCashDividends_019ef5c0-0bfa-79e5-afb1-836d181b8559" xlink:href="gbdc4-20250930.xsd#gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetProceedsFromNonCashDividends"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:to="loc_gbdc4_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetProceedsFromNonCashDividends_019ef5c0-0bfa-79e5-afb1-836d181b8559" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales_019ef5c0-0bfa-7d03-9f01-4441b7152757" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward_019ef5c0-0bfa-725a-be99-53f586f1d37f" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales_019ef5c0-0bfa-7d03-9f01-4441b7152757" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue_019ef5c0-0bfa-75d5-947b-f9e56e28103e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueRecurringBasisUnobservableInputReconciliationAssetGainLossStatementOfIncomeExtensibleList_019ef5c0-0bfa-7cd8-bbe5-edc763772128" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueRecurringBasisUnobservableInputReconciliationAssetGainLossStatementOfIncomeExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-724e-ba63-75121b680728" xlink:to="loc_us-gaap_FairValueRecurringBasisUnobservableInputReconciliationAssetGainLossStatementOfIncomeExtensibleList_019ef5c0-0bfa-7cd8-bbe5-edc763772128" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/FairValueMeasurementsScheduleofFairValueMeasurementInputsandValuationTechniquesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7056-aba8-94eb5e57c3cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-772e-ae5a-d0cce134dd8b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7056-aba8-94eb5e57c3cc" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-772e-ae5a-d0cce134dd8b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-72c9-8327-045bd6ec1b83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-772e-ae5a-d0cce134dd8b" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-72c9-8327-045bd6ec1b83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-72c9-8327-045bd6ec1b83_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-72c9-8327-045bd6ec1b83" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-72c9-8327-045bd6ec1b83_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7ebe-935d-f02b894b0bd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019ef5c0-0bfa-72c9-8327-045bd6ec1b83" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7ebe-935d-f02b894b0bd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019ef5c0-0bfa-7191-8d09-7af42c2753ed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019ef5c0-0bfa-7ebe-935d-f02b894b0bd4" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019ef5c0-0bfa-7191-8d09-7af42c2753ed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019ef5c0-0bfa-7fb4-82b8-be8bcecb8e67" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-772e-ae5a-d0cce134dd8b" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019ef5c0-0bfa-7fb4-82b8-be8bcecb8e67" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019ef5c0-0bfa-7fb4-82b8-be8bcecb8e67_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019ef5c0-0bfa-7fb4-82b8-be8bcecb8e67" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019ef5c0-0bfa-7fb4-82b8-be8bcecb8e67_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019ef5c0-0bfa-77a0-b589-0e8429cbec2d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019ef5c0-0bfa-7fb4-82b8-be8bcecb8e67" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019ef5c0-0bfa-77a0-b589-0e8429cbec2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritiesSeniorSecuredLoanMember_019ef5c0-0bfa-709e-a990-fe7e3bf6d441" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritiesSeniorSecuredLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019ef5c0-0bfa-77a0-b589-0e8429cbec2d" xlink:to="loc_gbdc4_DebtSecuritiesSeniorSecuredLoanMember_019ef5c0-0bfa-709e-a990-fe7e3bf6d441" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritiesOneStopLoansMember_019ef5c0-0bfa-7277-a485-d0aedb3d7e1a" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritiesOneStopLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019ef5c0-0bfa-77a0-b589-0e8429cbec2d" xlink:to="loc_gbdc4_DebtSecuritiesOneStopLoansMember_019ef5c0-0bfa-7277-a485-d0aedb3d7e1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritiesSubordinatedDebtAndSecondLienLoanMember_019ef5c0-0bfa-7eac-974b-c63185423084" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritiesSubordinatedDebtAndSecondLienLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019ef5c0-0bfa-77a0-b589-0e8429cbec2d" xlink:to="loc_gbdc4_DebtSecuritiesSubordinatedDebtAndSecondLienLoanMember_019ef5c0-0bfa-7eac-974b-c63185423084" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bfa-797a-a7d3-b3769e958699" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019ef5c0-0bfa-77a0-b589-0e8429cbec2d" xlink:to="loc_us-gaap_EquitySecuritiesMember_019ef5c0-0bfa-797a-a7d3-b3769e958699" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019ef5c0-0bfa-7512-9919-d31b2e9b2991" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-772e-ae5a-d0cce134dd8b" xlink:to="loc_srt_RangeAxis_019ef5c0-0bfa-7512-9919-d31b2e9b2991" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ef5c0-0bfa-7512-9919-d31b2e9b2991_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019ef5c0-0bfa-7512-9919-d31b2e9b2991" xlink:to="loc_srt_RangeMember_019ef5c0-0bfa-7512-9919-d31b2e9b2991_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ef5c0-0bfa-7f14-98d4-cfc327aa4f14" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019ef5c0-0bfa-7512-9919-d31b2e9b2991" xlink:to="loc_srt_RangeMember_019ef5c0-0bfa-7f14-98d4-cfc327aa4f14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019ef5c0-0bfa-707e-959a-9962377d93d2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ef5c0-0bfa-7f14-98d4-cfc327aa4f14" xlink:to="loc_srt_MinimumMember_019ef5c0-0bfa-707e-959a-9962377d93d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019ef5c0-0bfa-7644-8a29-d3aed09d4ebe" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ef5c0-0bfa-7f14-98d4-cfc327aa4f14" xlink:to="loc_srt_MaximumMember_019ef5c0-0bfa-7644-8a29-d3aed09d4ebe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_019ef5c0-0bfa-7b89-8448-06cae6437a1a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ef5c0-0bfa-7f14-98d4-cfc327aa4f14" xlink:to="loc_srt_WeightedAverageMember_019ef5c0-0bfa-7b89-8448-06cae6437a1a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_019ef5c0-0bfa-7265-82cc-9691d47e677a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-772e-ae5a-d0cce134dd8b" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_019ef5c0-0bfa-7265-82cc-9691d47e677a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_019ef5c0-0bfa-7265-82cc-9691d47e677a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_019ef5c0-0bfa-7265-82cc-9691d47e677a" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_019ef5c0-0bfa-7265-82cc-9691d47e677a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_019ef5c0-0bfa-7b42-a93a-2be8ff454af3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_019ef5c0-0bfa-7265-82cc-9691d47e677a" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_019ef5c0-0bfa-7b42-a93a-2be8ff454af3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_MeasurementInputMarketInterestRateMember_019ef5c0-0bfa-7bd4-8235-68aa8635cb17" xlink:href="gbdc4-20250930.xsd#gbdc4_MeasurementInputMarketInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019ef5c0-0bfa-7b42-a93a-2be8ff454af3" xlink:to="loc_gbdc4_MeasurementInputMarketInterestRateMember_019ef5c0-0bfa-7bd4-8235-68aa8635cb17" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputEbitdaMultipleMember_019ef5c0-0bfa-7524-947b-592872006207" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputEbitdaMultipleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019ef5c0-0bfa-7b42-a93a-2be8ff454af3" xlink:to="loc_us-gaap_MeasurementInputEbitdaMultipleMember_019ef5c0-0bfa-7524-947b-592872006207" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRevenueMultipleMember_019ef5c0-0bfa-7343-9736-08b612d4bf2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputRevenueMultipleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_019ef5c0-0bfa-7b42-a93a-2be8ff454af3" xlink:to="loc_us-gaap_MeasurementInputRevenueMultipleMember_019ef5c0-0bfa-7343-9736-08b612d4bf2e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueAxis_019ef5c0-0bfa-71bd-a117-8b7ba9b56870" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTable_019ef5c0-0bfa-772e-ae5a-d0cce134dd8b" xlink:to="loc_us-gaap_ValuationTechniqueAxis_019ef5c0-0bfa-71bd-a117-8b7ba9b56870" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_019ef5c0-0bfa-71bd-a117-8b7ba9b56870_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ValuationTechniqueAxis_019ef5c0-0bfa-71bd-a117-8b7ba9b56870" xlink:to="loc_us-gaap_ValuationTechniqueDomain_019ef5c0-0bfa-71bd-a117-8b7ba9b56870_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationTechniqueDomain_019ef5c0-0bfa-7501-b5f7-aa4ec78e7245" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationTechniqueDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ValuationTechniqueAxis_019ef5c0-0bfa-71bd-a117-8b7ba9b56870" xlink:to="loc_us-gaap_ValuationTechniqueDomain_019ef5c0-0bfa-7501-b5f7-aa4ec78e7245" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ValuationYieldAnalysisMember_019ef5c0-0bfa-71de-8415-f2612afb29c3" xlink:href="gbdc4-20250930.xsd#gbdc4_ValuationYieldAnalysisMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_019ef5c0-0bfa-7501-b5f7-aa4ec78e7245" xlink:to="loc_gbdc4_ValuationYieldAnalysisMember_019ef5c0-0bfa-71de-8415-f2612afb29c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ValuationTechniqueBrokerDealerBidsOrQuotesMember_019ef5c0-0bfa-704e-b428-aeaba35d1e9b" xlink:href="gbdc4-20250930.xsd#gbdc4_ValuationTechniqueBrokerDealerBidsOrQuotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_019ef5c0-0bfa-7501-b5f7-aa4ec78e7245" xlink:to="loc_gbdc4_ValuationTechniqueBrokerDealerBidsOrQuotesMember_019ef5c0-0bfa-704e-b428-aeaba35d1e9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ValuationMarketComparableCompanyMember_019ef5c0-0bfa-7566-8546-9fceb0f6f036" xlink:href="gbdc4-20250930.xsd#gbdc4_ValuationMarketComparableCompanyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ValuationTechniqueDomain_019ef5c0-0bfa-7501-b5f7-aa4ec78e7245" xlink:to="loc_gbdc4_ValuationMarketComparableCompanyMember_019ef5c0-0bfa-7566-8546-9fceb0f6f036" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-750b-b694-7caa7efdfeff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7056-aba8-94eb5e57c3cc" xlink:to="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-750b-b694-7caa7efdfeff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyInvestmentOwnedFairValueMeasurementInput_019ef5c0-0bfa-7276-a775-25f91742859c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyInvestmentOwnedFairValueMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems_019ef5c0-0bfa-7056-aba8-94eb5e57c3cc" xlink:to="loc_us-gaap_InvestmentCompanyInvestmentOwnedFairValueMeasurementInput_019ef5c0-0bfa-7276-a775-25f91742859c" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/BorrowingsNarrativeDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#BorrowingsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/BorrowingsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-71ec-a842-160a79ab1528" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-71ec-a842-160a79ab1528" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-71ec-a842-160a79ab1528_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-71ec-a842-160a79ab1528" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-71ec-a842-160a79ab1528_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-771e-8504-e55a55e8b3cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-71ec-a842-160a79ab1528" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-771e-8504-e55a55e8b3cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritization2025Member_019ef5c0-0bfa-70f1-a229-d495d37b520a" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritization2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-771e-8504-e55a55e8b3cc" xlink:to="loc_gbdc4_DebtSecuritization2025Member_019ef5c0-0bfa-70f1-a229-d495d37b520a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PNCFacilityMember_019ef5c0-0bfa-7c01-9c56-66dd35ed2ce1" xlink:href="gbdc4-20250930.xsd#gbdc4_PNCFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-771e-8504-e55a55e8b3cc" xlink:to="loc_gbdc4_PNCFacilityMember_019ef5c0-0bfa-7c01-9c56-66dd35ed2ce1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DBCreditFacilityMember_019ef5c0-0bfa-7908-b28a-339b88ef3fa1" xlink:href="gbdc4-20250930.xsd#gbdc4_DBCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-771e-8504-e55a55e8b3cc" xlink:to="loc_gbdc4_DBCreditFacilityMember_019ef5c0-0bfa-7908-b28a-339b88ef3fa1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BNPCreditFacilityMember_019ef5c0-0bfa-7b53-820a-a0c82add10f8" xlink:href="gbdc4-20250930.xsd#gbdc4_BNPCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-771e-8504-e55a55e8b3cc" xlink:to="loc_gbdc4_BNPCreditFacilityMember_019ef5c0-0bfa-7b53-820a-a0c82add10f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AdviserRevolverMember_019ef5c0-0bfa-7db8-b25e-3e13d0b4439e" xlink:href="gbdc4-20250930.xsd#gbdc4_AdviserRevolverMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-771e-8504-e55a55e8b3cc" xlink:to="loc_gbdc4_AdviserRevolverMember_019ef5c0-0bfa-7db8-b25e-3e13d0b4439e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritizationTypeAxis_019ef5c0-0bfa-7b38-b29a-978aebae38e7" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritizationTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:to="loc_gbdc4_DebtSecuritizationTypeAxis_019ef5c0-0bfa-7b38-b29a-978aebae38e7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7b38-b29a-978aebae38e7_default" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritizationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_gbdc4_DebtSecuritizationTypeAxis_019ef5c0-0bfa-7b38-b29a-978aebae38e7" xlink:to="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7b38-b29a-978aebae38e7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7090-bcdb-bf32a7079d6e" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritizationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_gbdc4_DebtSecuritizationTypeAxis_019ef5c0-0bfa-7b38-b29a-978aebae38e7" xlink:to="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7090-bcdb-bf32a7079d6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ClassA1SeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-7a9f-956e-0dc65517ba80" xlink:href="gbdc4-20250930.xsd#gbdc4_ClassA1SeniorSecuredFloatingRateNotesDue2037Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7090-bcdb-bf32a7079d6e" xlink:to="loc_gbdc4_ClassA1SeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-7a9f-956e-0dc65517ba80" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ClassA2SeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-796c-b5f1-ce87c85d5bb6" xlink:href="gbdc4-20250930.xsd#gbdc4_ClassA2SeniorSecuredFloatingRateNotesDue2037Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7090-bcdb-bf32a7079d6e" xlink:to="loc_gbdc4_ClassA2SeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-796c-b5f1-ce87c85d5bb6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ClassBSeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-7cc4-844a-ba36ac790a26" xlink:href="gbdc4-20250930.xsd#gbdc4_ClassBSeniorSecuredFloatingRateNotesDue2037Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7090-bcdb-bf32a7079d6e" xlink:to="loc_gbdc4_ClassBSeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-7cc4-844a-ba36ac790a26" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ClassCSeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-705a-b2f8-dd899d64be59" xlink:href="gbdc4-20250930.xsd#gbdc4_ClassCSeniorSecuredFloatingRateNotesDue2037Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7090-bcdb-bf32a7079d6e" xlink:to="loc_gbdc4_ClassCSeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-705a-b2f8-dd899d64be59" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019ef5c0-0bfa-7eda-b1d3-9b5d6c25f60f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019ef5c0-0bfa-7eda-b1d3-9b5d6c25f60f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ef5c0-0bfa-7eda-b1d3-9b5d6c25f60f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ef5c0-0bfa-7eda-b1d3-9b5d6c25f60f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ef5c0-0bfa-7eda-b1d3-9b5d6c25f60f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019ef5c0-0bfa-712b-b986-bf5d7165e519" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019ef5c0-0bfa-7eda-b1d3-9b5d6c25f60f" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019ef5c0-0bfa-712b-b986-bf5d7165e519" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubordinatedDebtMember_019ef5c0-0bfa-78fa-8c16-75f66a158ef6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubordinatedDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ef5c0-0bfa-712b-b986-bf5d7165e519" xlink:to="loc_us-gaap_SubordinatedDebtMember_019ef5c0-0bfa-78fa-8c16-75f66a158ef6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019ef5c0-0bfa-7276-8932-6afc51a94f6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019ef5c0-0bfa-712b-b986-bf5d7165e519" xlink:to="loc_us-gaap_LineOfCreditMember_019ef5c0-0bfa-7276-8932-6afc51a94f6e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019ef5c0-0bfa-76c7-b43e-895ace35dcb7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:to="loc_us-gaap_CreditFacilityAxis_019ef5c0-0bfa-76c7-b43e-895ace35dcb7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bfa-76c7-b43e-895ace35dcb7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019ef5c0-0bfa-76c7-b43e-895ace35dcb7" xlink:to="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bfa-76c7-b43e-895ace35dcb7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bfa-7ef2-8fb9-69d427265fd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019ef5c0-0bfa-76c7-b43e-895ace35dcb7" xlink:to="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bfa-7ef2-8fb9-69d427265fd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019ef5c0-0bfa-7eeb-a2d3-cb52fc197d1d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019ef5c0-0bfa-7ef2-8fb9-69d427265fd4" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019ef5c0-0bfa-7eeb-a2d3-cb52fc197d1d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019ef5c0-0bfa-7aae-b994-8c5a87c7bc9f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:to="loc_srt_RangeAxis_019ef5c0-0bfa-7aae-b994-8c5a87c7bc9f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ef5c0-0bfa-7aae-b994-8c5a87c7bc9f_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019ef5c0-0bfa-7aae-b994-8c5a87c7bc9f" xlink:to="loc_srt_RangeMember_019ef5c0-0bfa-7aae-b994-8c5a87c7bc9f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019ef5c0-0bfa-7b92-81b8-97afabdf22bf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019ef5c0-0bfa-7aae-b994-8c5a87c7bc9f" xlink:to="loc_srt_RangeMember_019ef5c0-0bfa-7b92-81b8-97afabdf22bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019ef5c0-0bfa-7e6a-bb32-2a0d0b257a61" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ef5c0-0bfa-7b92-81b8-97afabdf22bf" xlink:to="loc_srt_MinimumMember_019ef5c0-0bfa-7e6a-bb32-2a0d0b257a61" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019ef5c0-0bfa-736d-bbc4-df6c2dbed9f6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019ef5c0-0bfa-7b92-81b8-97afabdf22bf" xlink:to="loc_srt_MaximumMember_019ef5c0-0bfa-736d-bbc4-df6c2dbed9f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019ef5c0-0bfa-775f-bfa1-8d606100ab8e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:to="loc_us-gaap_VariableRateAxis_019ef5c0-0bfa-775f-bfa1-8d606100ab8e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019ef5c0-0bfa-775f-bfa1-8d606100ab8e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019ef5c0-0bfa-775f-bfa1-8d606100ab8e" xlink:to="loc_us-gaap_VariableRateDomain_019ef5c0-0bfa-775f-bfa1-8d606100ab8e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019ef5c0-0bfa-7107-b0c5-aae715da9042" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019ef5c0-0bfa-775f-bfa1-8d606100ab8e" xlink:to="loc_us-gaap_VariableRateDomain_019ef5c0-0bfa-7107-b0c5-aae715da9042" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_019ef5c0-0bfa-7080-9b1f-3b4b96bc06d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019ef5c0-0bfa-7107-b0c5-aae715da9042" xlink:to="loc_us-gaap_BaseRateMember_019ef5c0-0bfa-7080-9b1f-3b4b96bc06d9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CanadianOvernightRepoRateAverageMember_019ef5c0-0bfa-7da8-b5a3-3ca749245be9" xlink:href="gbdc4-20250930.xsd#gbdc4_CanadianOvernightRepoRateAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019ef5c0-0bfa-7107-b0c5-aae715da9042" xlink:to="loc_gbdc4_CanadianOvernightRepoRateAverageMember_019ef5c0-0bfa-7da8-b5a3-3ca749245be9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_SterlingOvernightIndexAverageMember_019ef5c0-0bfa-7aa2-9cd5-c32ec8efc9d7" xlink:href="gbdc4-20250930.xsd#gbdc4_SterlingOvernightIndexAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019ef5c0-0bfa-7107-b0c5-aae715da9042" xlink:to="loc_gbdc4_SterlingOvernightIndexAverageMember_019ef5c0-0bfa-7aa2-9cd5-c32ec8efc9d7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ShortTermApplicableFederalRateMember_019ef5c0-0bfa-7c3b-af67-4a894d9eda2e" xlink:href="gbdc4-20250930.xsd#gbdc4_ShortTermApplicableFederalRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019ef5c0-0bfa-7107-b0c5-aae715da9042" xlink:to="loc_gbdc4_ShortTermApplicableFederalRateMember_019ef5c0-0bfa-7c3b-af67-4a894d9eda2e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019ef5c0-0bfa-792d-9794-1b73d58283ef" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7e2d-bb14-c539302d1f25" xlink:to="loc_srt_StatementScenarioAxis_019ef5c0-0bfa-792d-9794-1b73d58283ef" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0bfa-792d-9794-1b73d58283ef_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019ef5c0-0bfa-792d-9794-1b73d58283ef" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0bfa-792d-9794-1b73d58283ef_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0bfa-7f2b-8a0c-19a25a944f61" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019ef5c0-0bfa-792d-9794-1b73d58283ef" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0bfa-7f2b-8a0c-19a25a944f61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_019ef5c0-0bfa-712c-a029-2ce1709ef255" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0bfa-7f2b-8a0c-19a25a944f61" xlink:to="loc_srt_ScenarioForecastMember_019ef5c0-0bfa-712c-a029-2ce1709ef255" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatioMinimumThreshold_019ef5c0-0bfa-70b9-9491-5daa0375615c" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatioMinimumThreshold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_gbdc4_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatioMinimumThreshold_019ef5c0-0bfa-70b9-9491-5daa0375615c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatio_019ef5c0-0bfa-7441-8c8b-2124a7d5fa89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatio_019ef5c0-0bfa-7441-8c8b-2124a7d5fa89" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019ef5c0-0bfa-71de-a3c1-7347666c9a8e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019ef5c0-0bfa-71de-a3c1-7347666c9a8e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ef5c0-0bfa-7392-9324-bb029815ffff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ef5c0-0bfa-7392-9324-bb029815ffff" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_NumberOfInvestmentPortfolioCompanies_019ef5c0-0bfa-7b17-bc5f-8e1f95ace015" xlink:href="gbdc4-20250930.xsd#gbdc4_NumberOfInvestmentPortfolioCompanies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_gbdc4_NumberOfInvestmentPortfolioCompanies_019ef5c0-0bfa-7b17-bc5f-8e1f95ace015" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-798f-a69a-14c5880d6529" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentOwnedAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_InvestmentOwnedAtFairValue_019ef5c0-0bfa-798f-a69a-14c5880d6529" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019ef5c0-0bfa-7c41-ae34-c6ea1bc2a92f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019ef5c0-0bfa-7c41-ae34-c6ea1bc2a92f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019ef5c0-0bfa-7eb8-b94e-93f4b0c908e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_LongTermDebt_019ef5c0-0bfa-7eb8-b94e-93f4b0c908e4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_019ef5c0-0bfa-70c6-95bc-02c15b8f44d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_DebtInstrumentTerm_019ef5c0-0bfa-70c6-95bc-02c15b8f44d0" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtInstrumentCreditSpreadAdjustment_019ef5c0-0bfa-74cd-b9ed-d2976e82a312" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtInstrumentCreditSpreadAdjustment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_gbdc4_DebtInstrumentCreditSpreadAdjustment_019ef5c0-0bfa-74cd-b9ed-d2976e82a312" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019ef5c0-0bfa-74e0-bd5d-d8b5f10e2cc5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage_019ef5c0-0bfa-74e0-bd5d-d8b5f10e2cc5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LineOfCreditFacilityPrepaymentFeePercentageYearOne_019ef5c0-0bfa-769a-9aef-7ede60633f64" xlink:href="gbdc4-20250930.xsd#gbdc4_LineOfCreditFacilityPrepaymentFeePercentageYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_gbdc4_LineOfCreditFacilityPrepaymentFeePercentageYearOne_019ef5c0-0bfa-769a-9aef-7ede60633f64" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LineOfCreditFacilityPrepaymentFeePercentageYearTwo_019ef5c0-0bfa-7faa-a1a4-666715b66586" xlink:href="gbdc4-20250930.xsd#gbdc4_LineOfCreditFacilityPrepaymentFeePercentageYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_gbdc4_LineOfCreditFacilityPrepaymentFeePercentageYearTwo_019ef5c0-0bfa-7faa-a1a4-666715b66586" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ef5c0-0bfa-7fa1-8e61-7f0f4fbf2d90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019ef5c0-0bfa-7fa1-8e61-7f0f4fbf2d90" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuritiesSoldUnderAgreementsToRepurchase_019ef5c0-0bfa-7c49-8459-f02474b44131" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesSoldUnderAgreementsToRepurchase"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_SecuritiesSoldUnderAgreementsToRepurchase_019ef5c0-0bfa-7c49-8459-f02474b44131" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShorttermDebtAverageOutstandingAmount_019ef5c0-0bfa-76a0-96e2-f5ccf4230b98" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShorttermDebtAverageOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_us-gaap_ShorttermDebtAverageOutstandingAmount_019ef5c0-0bfa-76a0-96e2-f5ccf4230b98" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtAnnualizedWeightedAverageInterestRate_019ef5c0-0bfa-7d16-a956-ad9bcd1196e7" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtAnnualizedWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7d90-a60e-c12c706f1239" xlink:to="loc_gbdc4_DebtAnnualizedWeightedAverageInterestRate_019ef5c0-0bfa-7d16-a956-ad9bcd1196e7" xlink:type="arc" order="16"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/BorrowingsScheduleofDebtSecuritizationDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#BorrowingsScheduleofDebtSecuritizationDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/BorrowingsScheduleofDebtSecuritizationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7b94-9b6d-e7f4875fbce4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7b94-9b6d-e7f4875fbce4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-7f06-9763-66ec7ccc08d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7b94-9b6d-e7f4875fbce4" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-7f06-9763-66ec7ccc08d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7f06-9763-66ec7ccc08d8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-7f06-9763-66ec7ccc08d8" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7f06-9763-66ec7ccc08d8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7672-a110-cb2f7a17237f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-7f06-9763-66ec7ccc08d8" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7672-a110-cb2f7a17237f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritization2025Member_019ef5c0-0bfa-7ef0-b03f-bf54b1c96b90" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritization2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7672-a110-cb2f7a17237f" xlink:to="loc_gbdc4_DebtSecuritization2025Member_019ef5c0-0bfa-7ef0-b03f-bf54b1c96b90" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PNCFacilityMember_019ef5c0-0bfa-7eec-ab25-4fb00f40afca" xlink:href="gbdc4-20250930.xsd#gbdc4_PNCFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7672-a110-cb2f7a17237f" xlink:to="loc_gbdc4_PNCFacilityMember_019ef5c0-0bfa-7eec-ab25-4fb00f40afca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DBCreditFacilityMember_019ef5c0-0bfa-7278-8fb8-d3de0c9b85b4" xlink:href="gbdc4-20250930.xsd#gbdc4_DBCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7672-a110-cb2f7a17237f" xlink:to="loc_gbdc4_DBCreditFacilityMember_019ef5c0-0bfa-7278-8fb8-d3de0c9b85b4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BNPCreditFacilityMember_019ef5c0-0bfa-7beb-b96d-06068d766dc8" xlink:href="gbdc4-20250930.xsd#gbdc4_BNPCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7672-a110-cb2f7a17237f" xlink:to="loc_gbdc4_BNPCreditFacilityMember_019ef5c0-0bfa-7beb-b96d-06068d766dc8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AdviserRevolverMember_019ef5c0-0bfa-73b9-b08c-d30979d80043" xlink:href="gbdc4-20250930.xsd#gbdc4_AdviserRevolverMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7672-a110-cb2f7a17237f" xlink:to="loc_gbdc4_AdviserRevolverMember_019ef5c0-0bfa-73b9-b08c-d30979d80043" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_OtherShortTermBorrowingsMember_019ef5c0-0bfa-7601-8643-0880d9f6d426" xlink:href="gbdc4-20250930.xsd#gbdc4_OtherShortTermBorrowingsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7672-a110-cb2f7a17237f" xlink:to="loc_gbdc4_OtherShortTermBorrowingsMember_019ef5c0-0bfa-7601-8643-0880d9f6d426" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_019ef5c0-0bfa-78c7-be1e-25cc9ea06b97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebtExcludingAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_019ef5c0-0bfa-78c7-be1e-25cc9ea06b97" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_FacilityFees_019ef5c0-0bfa-7e48-8a48-0863bf7a2126" xlink:href="gbdc4-20250930.xsd#gbdc4_FacilityFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_gbdc4_FacilityFees_019ef5c0-0bfa-7e48-8a48-0863bf7a2126" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_019ef5c0-0bfa-7f82-ae9f-85045aee97a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_019ef5c0-0bfa-7f82-ae9f-85045aee97a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_019ef5c0-0bfa-7ead-b4c2-be0d3b4c58a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_us-gaap_InterestExpenseDebt_019ef5c0-0bfa-7ead-b4c2-be0d3b4c58a3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_019ef5c0-0bfa-712e-8928-dde3535e9b94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_us-gaap_InterestPaidNet_019ef5c0-0bfa-712e-8928-dde3535e9b94" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtAnnualizedWeightedAverageInterestRate_019ef5c0-0bfa-7089-a5ad-3ffb1b6423b5" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtAnnualizedWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_gbdc4_DebtAnnualizedWeightedAverageInterestRate_019ef5c0-0bfa-7089-a5ad-3ffb1b6423b5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAverageAmountOutstanding_019ef5c0-0bfa-7d18-ada0-f561b1a27f28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAverageAmountOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_us-gaap_LongTermDebtAverageAmountOutstanding_019ef5c0-0bfa-7d18-ada0-f561b1a27f28" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShorttermDebtAverageOutstandingAmount_019ef5c0-0bfa-736e-a280-5ca8d9375cf0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShorttermDebtAverageOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7456-a622-e6c734daad83" xlink:to="loc_us-gaap_ShorttermDebtAverageOutstandingAmount_019ef5c0-0bfa-736e-a280-5ca8d9375cf0" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/BorrowingsScheduleofInterestRateandInterestDifferentialAnalysisDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#BorrowingsScheduleofInterestRateandInterestDifferentialAnalysisDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/BorrowingsScheduleofInterestRateandInterestDifferentialAnalysisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7730-b45c-3ed7052e09ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7926-bf31-0acc712acbf3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7730-b45c-3ed7052e09ab" xlink:to="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7926-bf31-0acc712acbf3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritizationTypeAxis_019ef5c0-0bfa-73b3-bacc-3ea0c75c8652" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritizationTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7926-bf31-0acc712acbf3" xlink:to="loc_gbdc4_DebtSecuritizationTypeAxis_019ef5c0-0bfa-73b3-bacc-3ea0c75c8652" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-73b3-bacc-3ea0c75c8652_default" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritizationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_gbdc4_DebtSecuritizationTypeAxis_019ef5c0-0bfa-73b3-bacc-3ea0c75c8652" xlink:to="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-73b3-bacc-3ea0c75c8652_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7865-8893-03b9877c9ee8" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritizationTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_gbdc4_DebtSecuritizationTypeAxis_019ef5c0-0bfa-73b3-bacc-3ea0c75c8652" xlink:to="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7865-8893-03b9877c9ee8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ClassA1SeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-7966-a765-afd70a51a374" xlink:href="gbdc4-20250930.xsd#gbdc4_ClassA1SeniorSecuredFloatingRateNotesDue2037Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7865-8893-03b9877c9ee8" xlink:to="loc_gbdc4_ClassA1SeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-7966-a765-afd70a51a374" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ClassA2SeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-711d-8e00-619efcceb7f2" xlink:href="gbdc4-20250930.xsd#gbdc4_ClassA2SeniorSecuredFloatingRateNotesDue2037Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7865-8893-03b9877c9ee8" xlink:to="loc_gbdc4_ClassA2SeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-711d-8e00-619efcceb7f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_ClassBSeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-7b45-8264-d243ae0a5e6d" xlink:href="gbdc4-20250930.xsd#gbdc4_ClassBSeniorSecuredFloatingRateNotesDue2037Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gbdc4_DebtSecuritizationTypeDomain_019ef5c0-0bfa-7865-8893-03b9877c9ee8" xlink:to="loc_gbdc4_ClassBSeniorSecuredFloatingRateNotesDue2037Member_019ef5c0-0bfa-7b45-8264-d243ae0a5e6d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-7f31-b382-ca67a222de19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-7926-bf31-0acc712acbf3" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-7f31-b382-ca67a222de19" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7f31-b382-ca67a222de19_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-7f31-b382-ca67a222de19" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7f31-b382-ca67a222de19_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7c2c-b2f9-34df27ab3815" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-7f31-b382-ca67a222de19" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7c2c-b2f9-34df27ab3815" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritization2025Member_019ef5c0-0bfa-774b-8f06-ab4035698ca0" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritization2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7c2c-b2f9-34df27ab3815" xlink:to="loc_gbdc4_DebtSecuritization2025Member_019ef5c0-0bfa-774b-8f06-ab4035698ca0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019ef5c0-0bfa-7b42-b03c-8dccfbec392b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7730-b45c-3ed7052e09ab" xlink:to="loc_us-gaap_LongTermDebt_019ef5c0-0bfa-7b42-b03c-8dccfbec392b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ef5c0-0bfa-75a8-9adc-f683333e475b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7730-b45c-3ed7052e09ab" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019ef5c0-0bfa-75a8-9adc-f683333e475b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/BorrowingsScheduleofMaturitiesofLongTermDebtDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#BorrowingsScheduleofMaturitiesofLongTermDebtDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/BorrowingsScheduleofMaturitiesofLongTermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7999-928f-bbf3c7db17ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-70de-8b9f-10cc181a0437" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7999-928f-bbf3c7db17ab" xlink:to="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-70de-8b9f-10cc181a0437" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-703e-935f-9fe14347a75f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019ef5c0-0bfa-70de-8b9f-10cc181a0437" xlink:to="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-703e-935f-9fe14347a75f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-703e-935f-9fe14347a75f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-703e-935f-9fe14347a75f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-703e-935f-9fe14347a75f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7293-a7e8-7e8d378e0bb1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019ef5c0-0bfa-703e-935f-9fe14347a75f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7293-a7e8-7e8d378e0bb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DBCreditFacilityMember_019ef5c0-0bfa-73a2-a59a-eba520b9ad9d" xlink:href="gbdc4-20250930.xsd#gbdc4_DBCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7293-a7e8-7e8d378e0bb1" xlink:to="loc_gbdc4_DBCreditFacilityMember_019ef5c0-0bfa-73a2-a59a-eba520b9ad9d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BNPCreditFacilityMember_019ef5c0-0bfa-70d8-8344-ef7615e93bd9" xlink:href="gbdc4-20250930.xsd#gbdc4_BNPCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7293-a7e8-7e8d378e0bb1" xlink:to="loc_gbdc4_BNPCreditFacilityMember_019ef5c0-0bfa-70d8-8344-ef7615e93bd9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritization2025Member_019ef5c0-0bfa-7ec4-bb48-df81d3f678dd" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritization2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019ef5c0-0bfa-7293-a7e8-7e8d378e0bb1" xlink:to="loc_gbdc4_DebtSecuritization2025Member_019ef5c0-0bfa-7ec4-bb48-df81d3f678dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019ef5c0-0bfa-7d18-ae91-d6d3fa28e7fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MaturitiesOfLongTermDebtAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019ef5c0-0bfa-7999-928f-bbf3c7db17ab" xlink:to="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019ef5c0-0bfa-7d18-ae91-d6d3fa28e7fb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019ef5c0-0bfa-77c1-bca5-9250a46f63f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019ef5c0-0bfa-7d18-ae91-d6d3fa28e7fb" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019ef5c0-0bfa-77c1-bca5-9250a46f63f6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LongTermDebtGrossMaturityLessThan1Year_019ef5c0-0bfa-72e7-bfce-d12456acbd08" xlink:href="gbdc4-20250930.xsd#gbdc4_LongTermDebtGrossMaturityLessThan1Year"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019ef5c0-0bfa-7d18-ae91-d6d3fa28e7fb" xlink:to="loc_gbdc4_LongTermDebtGrossMaturityLessThan1Year_019ef5c0-0bfa-72e7-bfce-d12456acbd08" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LongTermDebtGrossMaturity1Through3Years_019ef5c0-0bfa-7378-88e7-84afe650c7b8" xlink:href="gbdc4-20250930.xsd#gbdc4_LongTermDebtGrossMaturity1Through3Years"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019ef5c0-0bfa-7d18-ae91-d6d3fa28e7fb" xlink:to="loc_gbdc4_LongTermDebtGrossMaturity1Through3Years_019ef5c0-0bfa-7378-88e7-84afe650c7b8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LongTermDebtGrossMaturity3Through5Years_019ef5c0-0bfa-72a3-9e97-bd5f7bdfb803" xlink:href="gbdc4-20250930.xsd#gbdc4_LongTermDebtGrossMaturity3Through5Years"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019ef5c0-0bfa-7d18-ae91-d6d3fa28e7fb" xlink:to="loc_gbdc4_LongTermDebtGrossMaturity3Through5Years_019ef5c0-0bfa-72a3-9e97-bd5f7bdfb803" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LongTermDebtGrossMaturityMoreThan5Years_019ef5c0-0bfa-7636-9696-80d8a0f1622a" xlink:href="gbdc4-20250930.xsd#gbdc4_LongTermDebtGrossMaturityMoreThan5Years"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MaturitiesOfLongTermDebtAbstract_019ef5c0-0bfa-7d18-ae91-d6d3fa28e7fb" xlink:to="loc_gbdc4_LongTermDebtGrossMaturityMoreThan5Years_019ef5c0-0bfa-7636-9696-80d8a0f1622a" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/FederalIncomeTaxMattersNarrativeDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#FederalIncomeTaxMattersNarrativeDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/FederalIncomeTaxMattersNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentsOwnedFederalIncomeTaxNoteLineItems_019ef5c0-0bfa-7b48-bd64-5a97333c5d1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentsOwnedFederalIncomeTaxNoteLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalIncomeTaxNoteTable_019ef5c0-0bfa-7653-9ef0-a95ae62885bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FederalIncomeTaxNoteTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InvestmentsOwnedFederalIncomeTaxNoteLineItems_019ef5c0-0bfa-7b48-bd64-5a97333c5d1a" xlink:to="loc_us-gaap_FederalIncomeTaxNoteTable_019ef5c0-0bfa-7653-9ef0-a95ae62885bc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAxis_019ef5c0-0bfa-71e5-ae28-6fb2b7e5683d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FederalIncomeTaxNoteTable_019ef5c0-0bfa-7653-9ef0-a95ae62885bc" xlink:to="loc_us-gaap_TaxCreditCarryforwardAxis_019ef5c0-0bfa-71e5-ae28-6fb2b7e5683d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_019ef5c0-0bfa-71e5-ae28-6fb2b7e5683d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_019ef5c0-0bfa-71e5-ae28-6fb2b7e5683d" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_019ef5c0-0bfa-71e5-ae28-6fb2b7e5683d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_019ef5c0-0bfa-79e3-8134-c21306b9ae8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_019ef5c0-0bfa-71e5-ae28-6fb2b7e5683d" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_019ef5c0-0bfa-79e3-8134-c21306b9ae8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCreditMember_019ef5c0-0bfa-72cc-b347-18a230e9c1d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardNameDomain_019ef5c0-0bfa-79e3-8134-c21306b9ae8d" xlink:to="loc_us-gaap_InvestmentCreditMember_019ef5c0-0bfa-72cc-b347-18a230e9c1d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodAxis_019ef5c0-0bfa-7c91-becb-846cf18d69f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxPeriodAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FederalIncomeTaxNoteTable_019ef5c0-0bfa-7653-9ef0-a95ae62885bc" xlink:to="loc_us-gaap_TaxPeriodAxis_019ef5c0-0bfa-7c91-becb-846cf18d69f3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodDomain_019ef5c0-0bfa-7c91-becb-846cf18d69f3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxPeriodAxis_019ef5c0-0bfa-7c91-becb-846cf18d69f3" xlink:to="loc_us-gaap_TaxPeriodDomain_019ef5c0-0bfa-7c91-becb-846cf18d69f3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxPeriodDomain_019ef5c0-0bfa-7e40-b8a3-d6bb65c219f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxPeriodDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxPeriodAxis_019ef5c0-0bfa-7c91-becb-846cf18d69f3" xlink:to="loc_us-gaap_TaxPeriodDomain_019ef5c0-0bfa-7e40-b8a3-d6bb65c219f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxYear2025Member_019ef5c0-0bfa-72ec-bd0a-3a5124e01186" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxYear2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxPeriodDomain_019ef5c0-0bfa-7e40-b8a3-d6bb65c219f3" xlink:to="loc_us-gaap_TaxYear2025Member_019ef5c0-0bfa-72ec-bd0a-3a5124e01186" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyDistributableEarningsLossAccumulatedLongTermCapitalGainLoss_019ef5c0-0bfa-7bf2-8dff-05e40a5892d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyDistributableEarningsLossAccumulatedLongTermCapitalGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsOwnedFederalIncomeTaxNoteLineItems_019ef5c0-0bfa-7b48-bd64-5a97333c5d1a" xlink:to="loc_us-gaap_InvestmentCompanyDistributableEarningsLossAccumulatedLongTermCapitalGainLoss_019ef5c0-0bfa-7bf2-8dff-05e40a5892d9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxBasisOfInvestmentsCostForIncomeTaxPurposes_019ef5c0-0bfa-7e33-a683-1a643136d6d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxBasisOfInvestmentsCostForIncomeTaxPurposes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsOwnedFederalIncomeTaxNoteLineItems_019ef5c0-0bfa-7b48-bd64-5a97333c5d1a" xlink:to="loc_us-gaap_TaxBasisOfInvestmentsCostForIncomeTaxPurposes_019ef5c0-0bfa-7e33-a683-1a643136d6d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxBasisOfInvestmentsGrossUnrealizedAppreciation_019ef5c0-0bfa-7d56-be51-32bcd70e9e6e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxBasisOfInvestmentsGrossUnrealizedAppreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsOwnedFederalIncomeTaxNoteLineItems_019ef5c0-0bfa-7b48-bd64-5a97333c5d1a" xlink:to="loc_us-gaap_TaxBasisOfInvestmentsGrossUnrealizedAppreciation_019ef5c0-0bfa-7d56-be51-32bcd70e9e6e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxBasisOfInvestmentsGrossUnrealizedDepreciation_019ef5c0-0bfa-7b2b-b04e-1ea968583edb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxBasisOfInvestmentsGrossUnrealizedDepreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentsOwnedFederalIncomeTaxNoteLineItems_019ef5c0-0bfa-7b48-bd64-5a97333c5d1a" xlink:to="loc_us-gaap_TaxBasisOfInvestmentsGrossUnrealizedDepreciation_019ef5c0-0bfa-7b2b-b04e-1ea968583edb" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019ef5c0-0bfa-7441-a26c-ca8d0b5f94d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019ef5c0-0bfa-7e7a-9dae-49e915ab6e85" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019ef5c0-0bfa-7441-a26c-ca8d0b5f94d1" xlink:to="loc_us-gaap_LossContingenciesTable_019ef5c0-0bfa-7e7a-9dae-49e915ab6e85" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityAxis_019ef5c0-0bfa-7e09-9f8d-c13065fcdb65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialSupportToNonconsolidatedLegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019ef5c0-0bfa-7e7a-9dae-49e915ab6e85" xlink:to="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityAxis_019ef5c0-0bfa-7e09-9f8d-c13065fcdb65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityDomain_019ef5c0-0bfa-7e09-9f8d-c13065fcdb65_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialSupportToNonconsolidatedLegalEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityAxis_019ef5c0-0bfa-7e09-9f8d-c13065fcdb65" xlink:to="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityDomain_019ef5c0-0bfa-7e09-9f8d-c13065fcdb65_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityDomain_019ef5c0-0bfa-7604-b531-4028e9eb1d92" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialSupportToNonconsolidatedLegalEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityAxis_019ef5c0-0bfa-7e09-9f8d-c13065fcdb65" xlink:to="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityDomain_019ef5c0-0bfa-7604-b531-4028e9eb1d92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_UndrawnRevolverMember_019ef5c0-0bfa-7049-9f05-dd63c39a18f1" xlink:href="gbdc4-20250930.xsd#gbdc4_UndrawnRevolverMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialSupportToNonconsolidatedLegalEntityDomain_019ef5c0-0bfa-7604-b531-4028e9eb1d92" xlink:to="loc_gbdc4_UndrawnRevolverMember_019ef5c0-0bfa-7049-9f05-dd63c39a18f1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyFinancialCommitmentToInvesteeFutureAmount_019ef5c0-0bfa-7c6d-b4c8-187ffd72b02c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyFinancialCommitmentToInvesteeFutureAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019ef5c0-0bfa-7441-a26c-ca8d0b5f94d1" xlink:to="loc_us-gaap_InvestmentCompanyFinancialCommitmentToInvesteeFutureAmount_019ef5c0-0bfa-7c6d-b4c8-187ffd72b02c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/FinancialHighlightsDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#FinancialHighlightsDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/FinancialHighlightsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyFinancialHighlightsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyFinancialHighlightsTable_019ef5c0-0bfa-7b1b-a159-a29a55fc20b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyFinancialHighlightsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_InvestmentCompanyFinancialHighlightsTable_019ef5c0-0bfa-7b1b-a159-a29a55fc20b3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis_019ef5c0-0bfa-7fee-9e99-3177b02b7428" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsTable_019ef5c0-0bfa-7b1b-a159-a29a55fc20b3" xlink:to="loc_us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis_019ef5c0-0bfa-7fee-9e99-3177b02b7428" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DistributionTypeDomain_019ef5c0-0bfa-7fee-9e99-3177b02b7428_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DistributionTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis_019ef5c0-0bfa-7fee-9e99-3177b02b7428" xlink:to="loc_us-gaap_DistributionTypeDomain_019ef5c0-0bfa-7fee-9e99-3177b02b7428_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DistributionTypeDomain_019ef5c0-0bfa-7498-832a-f655aa87f9c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DistributionTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DistributionsMadeToMemberOrLimitedPartnerByDistributionTypeAxis_019ef5c0-0bfa-7fee-9e99-3177b02b7428" xlink:to="loc_us-gaap_DistributionTypeDomain_019ef5c0-0bfa-7498-832a-f655aa87f9c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeMember_019ef5c0-0bfa-7d21-bc87-2cfb9be585ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DistributionTypeDomain_019ef5c0-0bfa-7498-832a-f655aa87f9c9" xlink:to="loc_us-gaap_InvestmentIncomeMember_019ef5c0-0bfa-7d21-bc87-2cfb9be585ae" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_CapitalGainMember_019ef5c0-0bfa-7096-b10c-8c3ca9a92484" xlink:href="gbdc4-20250930.xsd#gbdc4_CapitalGainMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DistributionTypeDomain_019ef5c0-0bfa-7498-832a-f655aa87f9c9" xlink:to="loc_gbdc4_CapitalGainMember_019ef5c0-0bfa-7096-b10c-8c3ca9a92484" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyFinancialHighlightsRollForward_019ef5c0-0bfa-765f-bf83-b96d469d518a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyFinancialHighlightsRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_InvestmentCompanyFinancialHighlightsRollForward_019ef5c0-0bfa-765f-bf83-b96d469d518a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetAssetValuePerShare_019ef5c0-0bfa-769c-add3-4bf64e6bc8a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetAssetValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsRollForward_019ef5c0-0bfa-765f-bf83-b96d469d518a" xlink:to="loc_us-gaap_NetAssetValuePerShare_019ef5c0-0bfa-769c-add3-4bf64e6bc8a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyDistributionToShareholdersPerShare_019ef5c0-0bfa-7395-94b4-9ddb02e82ff2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyDistributionToShareholdersPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsRollForward_019ef5c0-0bfa-765f-bf83-b96d469d518a" xlink:to="loc_us-gaap_InvestmentCompanyDistributionToShareholdersPerShare_019ef5c0-0bfa-7395-94b4-9ddb02e82ff2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyInvestmentIncomeLossPerShare_019ef5c0-0bfa-7326-8d64-b4be95013e57" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyInvestmentIncomeLossPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsRollForward_019ef5c0-0bfa-765f-bf83-b96d469d518a" xlink:to="loc_us-gaap_InvestmentCompanyInvestmentIncomeLossPerShare_019ef5c0-0bfa-7326-8d64-b4be95013e57" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyRealizedGainLossOnInvestmentPerShare_019ef5c0-0bfa-7216-9ef9-0473c422ff9f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyRealizedGainLossOnInvestmentPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsRollForward_019ef5c0-0bfa-765f-bf83-b96d469d518a" xlink:to="loc_us-gaap_InvestmentCompanyRealizedGainLossOnInvestmentPerShare_019ef5c0-0bfa-7216-9ef9-0473c422ff9f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyUnrealizedGainLossOnInvestmentPerShare_019ef5c0-0bfa-79a3-9f73-b17785e30c58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyUnrealizedGainLossOnInvestmentPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsRollForward_019ef5c0-0bfa-765f-bf83-b96d469d518a" xlink:to="loc_us-gaap_InvestmentCompanyUnrealizedGainLossOnInvestmentPerShare_019ef5c0-0bfa-79a3-9f73-b17785e30c58" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetAssetValuePerShare_019ef5c0-0bfa-7964-87b9-4f2b0f884d30" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetAssetValuePerShare"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyTotalReturnMarketValue_019ef5c0-0bfa-7047-b0ed-82c207726520" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyTotalReturnMarketValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_InvestmentCompanyTotalReturnMarketValue_019ef5c0-0bfa-7047-b0ed-82c207726520" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ef5c0-0bfa-7dd4-b651-b6de9b069dce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019ef5c0-0bfa-7dd4-b651-b6de9b069dce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyInvestmentIncomeLossRatio_019ef5c0-0bfa-7538-90ed-8b649c82401c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyInvestmentIncomeLossRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_InvestmentCompanyInvestmentIncomeLossRatio_019ef5c0-0bfa-7538-90ed-8b649c82401c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyTotalExpensesToAverageNetAssets_019ef5c0-0bfa-7cc2-9f05-1c6b13d663ec" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyTotalExpensesToAverageNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_InvestmentCompanyTotalExpensesToAverageNetAssets_019ef5c0-0bfa-7cc2-9f05-1c6b13d663ec" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyManagementFeeWaiverRatioToAverageNetAssets_019ef5c0-0bfa-7138-b842-2bb6b237d8ff" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyManagementFeeWaiverRatioToAverageNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_InvestmentCompanyManagementFeeWaiverRatioToAverageNetAssets_019ef5c0-0bfa-7138-b842-2bb6b237d8ff" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyIncentiveFeeWaiverToAverageNetAssets_019ef5c0-0bfa-7ae8-ae46-e7bbbd8a68b8" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyIncentiveFeeWaiverToAverageNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_InvestmentCompanyIncentiveFeeWaiverToAverageNetAssets_019ef5c0-0bfa-7ae8-ae46-e7bbbd8a68b8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyOperatingExpenseWaiverToAverageNetAssets_019ef5c0-0bfa-7d31-90de-874c8de47664" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyOperatingExpenseWaiverToAverageNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_InvestmentCompanyOperatingExpenseWaiverToAverageNetAssets_019ef5c0-0bfa-7d31-90de-874c8de47664" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyIncentiveFeeToAverageNetAssets_019ef5c0-0bfa-7588-aa04-37387e240b77" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyIncentiveFeeToAverageNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_InvestmentCompanyIncentiveFeeToAverageNetAssets_019ef5c0-0bfa-7588-aa04-37387e240b77" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyExciseTaxToAverageNetAssets_019ef5c0-0bfa-7dd9-9d46-42b8cd529425" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyExciseTaxToAverageNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_InvestmentCompanyExciseTaxToAverageNetAssets_019ef5c0-0bfa-7dd9-9d46-42b8cd529425" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyNetExpensesToAverageNetAssets_019ef5c0-0bfa-7aeb-bf6c-92c02cae5659" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyNetExpensesToAverageNetAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_InvestmentCompanyNetExpensesToAverageNetAssets_019ef5c0-0bfa-7aeb-bf6c-92c02cae5659" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanyExpenseRatioBeforeIncentiveFees_019ef5c0-0bfa-7038-bdad-3052f0978c7e" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanyExpenseRatioBeforeIncentiveFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_InvestmentCompanyExpenseRatioBeforeIncentiveFees_019ef5c0-0bfa-7038-bdad-3052f0978c7e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyTotalReturn_019ef5c0-0bfa-7dee-bd1d-329e16855009" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyTotalReturn"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_InvestmentCompanyTotalReturn_019ef5c0-0bfa-7dee-bd1d-329e16855009" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019ef5c0-0bfa-7e8f-accb-3c38fcbc5bfa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_StockholdersEquity_019ef5c0-0bfa-7e8f-accb-3c38fcbc5bfa" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAverageAmountOutstanding_019ef5c0-0bfa-7076-b281-f15d9dc251bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAverageAmountOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_LongTermDebtAverageAmountOutstanding_019ef5c0-0bfa-7076-b281-f15d9dc251bf" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_LongTermDebtAverageAmountOutstandingPerShare_019ef5c0-0bfa-74be-8dec-9a802dfa0c7e" xlink:href="gbdc4-20250930.xsd#gbdc4_LongTermDebtAverageAmountOutstandingPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_LongTermDebtAverageAmountOutstandingPerShare_019ef5c0-0bfa-74be-8dec-9a802dfa0c7e" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyPortfolioTurnover_019ef5c0-0bfa-7934-b8cf-c2315e0d45f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyPortfolioTurnover"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_InvestmentCompanyPortfolioTurnover_019ef5c0-0bfa-7934-b8cf-c2315e0d45f8" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatio_019ef5c0-0bfa-7c44-8de5-b9b2308a359a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_us-gaap_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatio_019ef5c0-0bfa-7c44-8de5-b9b2308a359a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatioPerUnit_019ef5c0-0bfa-7a87-90bb-af080e960889" xlink:href="gbdc4-20250930.xsd#gbdc4_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatioPerUnit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InvestmentCompanyFinancialHighlightsLineItems_019ef5c0-0bfa-705e-b792-817800fa7362" xlink:to="loc_gbdc4_InvestmentCompanySeniorSecurityIndebtednessAssetCoverageRatioPerUnit_019ef5c0-0bfa-7a87-90bb-af080e960889" xlink:type="arc" order="18"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/DividendsandDistributionsScheduleofDividendsDeclarationsandDistributionsDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#DividendsandDistributionsScheduleofDividendsDeclarationsandDistributionsDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/DividendsandDistributionsScheduleofDividendsDeclarationsandDistributionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-72cb-9901-4e55a3920e86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ef5c0-0cd0-7a89-8425-0818a472d019" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-72cb-9901-4e55a3920e86" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ef5c0-0cd0-7a89-8425-0818a472d019" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsAxis_019ef5c0-0cd0-750b-b15e-9c21a1ffc408" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ef5c0-0cd0-7a89-8425-0818a472d019" xlink:to="loc_us-gaap_DividendsAxis_019ef5c0-0cd0-750b-b15e-9c21a1ffc408" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-750b-b15e-9c21a1ffc408_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DividendsAxis_019ef5c0-0cd0-750b-b15e-9c21a1ffc408" xlink:to="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-750b-b15e-9c21a1ffc408_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DividendsAxis_019ef5c0-0cd0-750b-b15e-9c21a1ffc408" xlink:to="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M1DividendsMember_019ef5c0-0cd0-7ae0-8156-500d3306c737" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M1DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M1DividendsMember_019ef5c0-0cd0-7ae0-8156-500d3306c737" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M2DividendsMember_019ef5c0-0cd0-71a0-9cac-aaadd06c4dd4" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M2DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M2DividendsMember_019ef5c0-0cd0-71a0-9cac-aaadd06c4dd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M3DividendsMember_019ef5c0-0cd0-7bf8-a9cb-2148cee26c9d" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M3DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M3DividendsMember_019ef5c0-0cd0-7bf8-a9cb-2148cee26c9d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M4DividendsMember_019ef5c0-0cd0-73cd-bf0e-82e6c0c0da57" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M4DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M4DividendsMember_019ef5c0-0cd0-73cd-bf0e-82e6c0c0da57" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M5DividendsMember_019ef5c0-0cd0-7308-a133-5e0a7a910e5c" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M5DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M5DividendsMember_019ef5c0-0cd0-7308-a133-5e0a7a910e5c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M6DividendsMember_019ef5c0-0cd0-7de5-9304-0d147092229b" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M6DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M6DividendsMember_019ef5c0-0cd0-7de5-9304-0d147092229b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M7DividendsMember_019ef5c0-0cd0-77c1-b14d-2bd7dbb902f7" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M7DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M7DividendsMember_019ef5c0-0cd0-77c1-b14d-2bd7dbb902f7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M8DividendsMember_019ef5c0-0cd0-72e7-9962-9fa5b175008d" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M8DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M8DividendsMember_019ef5c0-0cd0-72e7-9962-9fa5b175008d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M9DividendsMember_019ef5c0-0cd0-76c8-9071-1e08196e0395" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M9DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M9DividendsMember_019ef5c0-0cd0-76c8-9071-1e08196e0395" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M10DividendsMember_019ef5c0-0cd0-732e-8bcc-1e9565b22d42" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M10DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M10DividendsMember_019ef5c0-0cd0-732e-8bcc-1e9565b22d42" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M11DividendsMember_019ef5c0-0cd0-7ad2-addf-fa2458be816b" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M11DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M11DividendsMember_019ef5c0-0cd0-7ad2-addf-fa2458be816b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M12DividendsMember_019ef5c0-0cd0-7b20-87f6-339b22f8bfb0" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M12DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2025M12DividendsMember_019ef5c0-0cd0-7b20-87f6-339b22f8bfb0" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M1DividendsMember_019ef5c0-0cd0-7405-ab0a-d09ff584303b" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M1DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M1DividendsMember_019ef5c0-0cd0-7405-ab0a-d09ff584303b" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M2DividendsMember_019ef5c0-0cd0-7711-a80d-1282469d50e6" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M2DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M2DividendsMember_019ef5c0-0cd0-7711-a80d-1282469d50e6" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M3DividendsMember_019ef5c0-0cd0-7b1d-9b98-7177779d126c" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M3DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M3DividendsMember_019ef5c0-0cd0-7b1d-9b98-7177779d126c" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M4DividendsMember_019ef5c0-0cd0-7ccd-8bf3-77bc95e3487b" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M4DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M4DividendsMember_019ef5c0-0cd0-7ccd-8bf3-77bc95e3487b" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M5DividendsMember_019ef5c0-0cd0-7930-a63a-a5319a9666d4" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M5DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M5DividendsMember_019ef5c0-0cd0-7930-a63a-a5319a9666d4" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M6DividendsMember_019ef5c0-0cd0-718c-9478-4d9c2b46350a" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M6DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M6DividendsMember_019ef5c0-0cd0-718c-9478-4d9c2b46350a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M7DividendsMember_019ef5c0-0cd0-734f-8bcc-9e9164852023" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M7DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M7DividendsMember_019ef5c0-0cd0-734f-8bcc-9e9164852023" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M8DividendsMember_019ef5c0-0cd0-79cf-a554-e69f42a82349" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M8DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M8DividendsMember_019ef5c0-0cd0-79cf-a554-e69f42a82349" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M9DividendsMember_019ef5c0-0cd0-7844-b84a-9157a493ec2a" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M9DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M9DividendsMember_019ef5c0-0cd0-7844-b84a-9157a493ec2a" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M10DividendsMember_019ef5c0-0cd0-7289-a49f-5a2bfa48028f" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M10DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M10DividendsMember_019ef5c0-0cd0-7289-a49f-5a2bfa48028f" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M11DividendsMember_019ef5c0-0cd0-70d9-9934-73b94fc09e71" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M11DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M11DividendsMember_019ef5c0-0cd0-70d9-9934-73b94fc09e71" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M12DividendsMember_019ef5c0-0cd0-7975-969d-6cbf7684b742" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M12DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2024M12DividendsMember_019ef5c0-0cd0-7975-969d-6cbf7684b742" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M1DividendsMember_019ef5c0-0cd0-7cb1-8ee9-4bd4d9672dff" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M1DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M1DividendsMember_019ef5c0-0cd0-7cb1-8ee9-4bd4d9672dff" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M2DividendsMember_019ef5c0-0cd0-7242-afbc-7b15a4c30efd" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M2DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M2DividendsMember_019ef5c0-0cd0-7242-afbc-7b15a4c30efd" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M3DividendsMember_019ef5c0-0cd0-76f1-a122-45400c516795" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M3DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M3DividendsMember_019ef5c0-0cd0-76f1-a122-45400c516795" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M4DividendsMember_019ef5c0-0cd0-7c0c-9aa2-17cd303ddf83" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M4DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M4DividendsMember_019ef5c0-0cd0-7c0c-9aa2-17cd303ddf83" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M5DividendsMember_019ef5c0-0cd0-777f-990a-0701fd0d43e0" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M5DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M5DividendsMember_019ef5c0-0cd0-777f-990a-0701fd0d43e0" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M6DividendsMember_019ef5c0-0cd0-7ceb-86e7-98b231e987cb" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M6DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M6DividendsMember_019ef5c0-0cd0-7ceb-86e7-98b231e987cb" xlink:type="arc" order="29"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M7DividendsMember_019ef5c0-0cd0-7583-9260-3585bbd51b55" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M7DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M7DividendsMember_019ef5c0-0cd0-7583-9260-3585bbd51b55" xlink:type="arc" order="30"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M8DividendsMember_019ef5c0-0cd0-789f-be97-be937bcb2aaf" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M8DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M8DividendsMember_019ef5c0-0cd0-789f-be97-be937bcb2aaf" xlink:type="arc" order="31"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M9DividendsMember_019ef5c0-0cd0-7659-bc43-852a6521aa1d" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M9DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M9DividendsMember_019ef5c0-0cd0-7659-bc43-852a6521aa1d" xlink:type="arc" order="32"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M10DividendsMember_019ef5c0-0cd0-7235-ab05-a251f071124f" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M10DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M10DividendsMember_019ef5c0-0cd0-7235-ab05-a251f071124f" xlink:type="arc" order="33"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M11DividendsMember_019ef5c0-0cd0-7337-bef2-189cec8310ee" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M11DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M11DividendsMember_019ef5c0-0cd0-7337-bef2-189cec8310ee" xlink:type="arc" order="34"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M12DividendsMember_019ef5c0-0cd0-79e2-862b-0324d501e5b5" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M12DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-70df-9ea1-0afb3dbf932c" xlink:to="loc_gbdc4_O2023M12DividendsMember_019ef5c0-0cd0-79e2-862b-0324d501e5b5" xlink:type="arc" order="35"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019ef5c0-0cd0-7b35-85c7-7b2700122d05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-72cb-9901-4e55a3920e86" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019ef5c0-0cd0-7b35-85c7-7b2700122d05" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyDistributionToShareholdersPerShare_019ef5c0-0cd0-7ec4-950e-5553ada18cb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyDistributionToShareholdersPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-72cb-9901-4e55a3920e86" xlink:to="loc_us-gaap_InvestmentCompanyDistributionToShareholdersPerShare_019ef5c0-0cd0-7ec4-950e-5553ada18cb6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyDividendDistribution_019ef5c0-0cd0-7e58-98e0-495ab4d24822" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyDividendDistribution"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-72cb-9901-4e55a3920e86" xlink:to="loc_us-gaap_InvestmentCompanyDividendDistribution_019ef5c0-0cd0-7e58-98e0-495ab4d24822" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/DividendsandDistributionsScheduleofSummarizedDividendReinvestmentPlanInformationDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#DividendsandDistributionsScheduleofSummarizedDividendReinvestmentPlanInformationDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/DividendsandDistributionsScheduleofSummarizedDividendReinvestmentPlanInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-7bef-ab25-929cb7874b32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ef5c0-0cd0-7774-a9e6-38e24839b411" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-7bef-ab25-929cb7874b32" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ef5c0-0cd0-7774-a9e6-38e24839b411" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsAxis_019ef5c0-0cd0-75d9-9819-8471e13f9d34" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019ef5c0-0cd0-7774-a9e6-38e24839b411" xlink:to="loc_us-gaap_DividendsAxis_019ef5c0-0cd0-75d9-9819-8471e13f9d34" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-75d9-9819-8471e13f9d34_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DividendsAxis_019ef5c0-0cd0-75d9-9819-8471e13f9d34" xlink:to="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-75d9-9819-8471e13f9d34_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DividendsAxis_019ef5c0-0cd0-75d9-9819-8471e13f9d34" xlink:to="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M2DividendsMember_019ef5c0-0cd0-7fc1-b16b-53708ee41f6a" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M2DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M2DividendsMember_019ef5c0-0cd0-7fc1-b16b-53708ee41f6a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M31DividendsMember_019ef5c0-0cd0-73f4-bef0-affc734bfd4b" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M31DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M31DividendsMember_019ef5c0-0cd0-73f4-bef0-affc734bfd4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M32DividendsMember_019ef5c0-0cd0-7d13-b117-f1e9e8975325" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M32DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M32DividendsMember_019ef5c0-0cd0-7d13-b117-f1e9e8975325" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M51DividendsMember_019ef5c0-0cd0-77ed-83b3-afc0c345aed8" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M51DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M51DividendsMember_019ef5c0-0cd0-77ed-83b3-afc0c345aed8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M4DividendsMember_019ef5c0-0cd0-7baf-b71b-f16e608d9700" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M4DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M4DividendsMember_019ef5c0-0cd0-7baf-b71b-f16e608d9700" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M8DividendsMember_019ef5c0-0cd0-7578-97cd-0350a4eee125" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M8DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M8DividendsMember_019ef5c0-0cd0-7578-97cd-0350a4eee125" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M52DividendsMember_019ef5c0-0cd0-7d03-9d11-7f93b4e23643" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M52DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M52DividendsMember_019ef5c0-0cd0-7d03-9d11-7f93b4e23643" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M11DividendsMember_019ef5c0-0cd0-740d-aad7-884ef01d79ae" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M11DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M11DividendsMember_019ef5c0-0cd0-740d-aad7-884ef01d79ae" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2025M12DividendsMember_019ef5c0-0cd0-7528-b973-ff8469f8b6b8" xlink:href="gbdc4-20250930.xsd#gbdc4_O2025M12DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2025M12DividendsMember_019ef5c0-0cd0-7528-b973-ff8469f8b6b8" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M2DividendsMember_019ef5c0-0cd0-796f-820f-117d0ac47c4f" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M2DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2024M2DividendsMember_019ef5c0-0cd0-796f-820f-117d0ac47c4f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M3DividendsMember_019ef5c0-0cd0-7ffa-839d-623a61d44ffc" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M3DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2024M3DividendsMember_019ef5c0-0cd0-7ffa-839d-623a61d44ffc" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M5DividendsMember_019ef5c0-0cd0-7f15-9360-b1efb4b3b92e" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M5DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2024M5DividendsMember_019ef5c0-0cd0-7f15-9360-b1efb4b3b92e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M6DividendsMember_019ef5c0-0cd0-784e-80c2-2883ca7e799c" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M6DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2024M6DividendsMember_019ef5c0-0cd0-784e-80c2-2883ca7e799c" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M8DividendsMember_019ef5c0-0cd0-7a8c-aa1f-efe2808a3738" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M8DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2024M8DividendsMember_019ef5c0-0cd0-7a8c-aa1f-efe2808a3738" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M9DividendsMember_019ef5c0-0cd0-7a25-91c7-d78598155936" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M9DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2024M9DividendsMember_019ef5c0-0cd0-7a25-91c7-d78598155936" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M11DividendsMember_019ef5c0-0cd0-78b2-9f98-d1ada7802d47" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M11DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2024M11DividendsMember_019ef5c0-0cd0-78b2-9f98-d1ada7802d47" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2024M12DividendsMember_019ef5c0-0cd0-7e20-813c-1aee74e63fb3" xlink:href="gbdc4-20250930.xsd#gbdc4_O2024M12DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2024M12DividendsMember_019ef5c0-0cd0-7e20-813c-1aee74e63fb3" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M2DividendsMember_019ef5c0-0cd0-7a68-ad0e-3edaae0e21d7" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M2DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2023M2DividendsMember_019ef5c0-0cd0-7a68-ad0e-3edaae0e21d7" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M3DividendsMember_019ef5c0-0cd0-7c08-b84b-f26bc38c29a5" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M3DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2023M3DividendsMember_019ef5c0-0cd0-7c08-b84b-f26bc38c29a5" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M61DividendsMember_019ef5c0-0cd0-7623-8a1d-9a419a560f31" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M61DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2023M61DividendsMember_019ef5c0-0cd0-7623-8a1d-9a419a560f31" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M62DividendsMember_019ef5c0-0cd0-7c51-85e2-a588687f41c1" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M62DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2023M62DividendsMember_019ef5c0-0cd0-7c51-85e2-a588687f41c1" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M8DividendsMember_019ef5c0-0cd0-71c0-b232-9ff9c94e0bfc" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M8DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2023M8DividendsMember_019ef5c0-0cd0-71c0-b232-9ff9c94e0bfc" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M9DividendsMember_019ef5c0-0cd0-7033-be23-f7c1e32ba592" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M9DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2023M9DividendsMember_019ef5c0-0cd0-7033-be23-f7c1e32ba592" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M11DividendsMember_019ef5c0-0cd0-7461-aa72-3051bf954c35" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M11DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2023M11DividendsMember_019ef5c0-0cd0-7461-aa72-3051bf954c35" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_O2023M12DividendsMember_019ef5c0-0cd0-7e8e-88e9-dfb1db8ce632" xlink:href="gbdc4-20250930.xsd#gbdc4_O2023M12DividendsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DividendsDomain_019ef5c0-0cd0-72f9-8d89-58b19447b556" xlink:to="loc_gbdc4_O2023M12DividendsMember_019ef5c0-0cd0-7e8e-88e9-dfb1db8ce632" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan_019ef5c0-0cd0-7381-978e-06991425f56f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-7bef-ab25-929cb7874b32" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan_019ef5c0-0cd0-7381-978e-06991425f56f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetAssetValuePerShare_019ef5c0-0cd0-7cc3-9daa-7c26a111c89c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetAssetValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-7bef-ab25-929cb7874b32" xlink:to="loc_us-gaap_NetAssetValuePerShare_019ef5c0-0cd0-7cc3-9daa-7c26a111c89c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan_019ef5c0-0cd0-7dc5-96ad-aa64758a551f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019ef5c0-0cd0-7bef-ab25-929cb7874b32" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueDividendReinvestmentPlan_019ef5c0-0cd0-7dc5-96ad-aa64758a551f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/SubsequentEventsNarrativeDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#SubsequentEventsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/SubsequentEventsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019ef5c0-0cd0-738f-a648-98b93130dc4b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019ef5c0-0cd0-7d48-900a-e589d1315f01" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019ef5c0-0cd0-738f-a648-98b93130dc4b" xlink:to="loc_us-gaap_SubsequentEventTable_019ef5c0-0cd0-7d48-900a-e589d1315f01" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019ef5c0-0cd0-7823-a225-e5c9e3d8fd40" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019ef5c0-0cd0-7d48-900a-e589d1315f01" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019ef5c0-0cd0-7823-a225-e5c9e3d8fd40" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-7823-a225-e5c9e3d8fd40_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019ef5c0-0cd0-7823-a225-e5c9e3d8fd40" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-7823-a225-e5c9e3d8fd40_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-75e4-9205-94be47660c58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019ef5c0-0cd0-7823-a225-e5c9e3d8fd40" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-75e4-9205-94be47660c58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019ef5c0-0cd0-7de6-a1c6-06c6853ee3c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-75e4-9205-94be47660c58" xlink:to="loc_us-gaap_SubsequentEventMember_019ef5c0-0cd0-7de6-a1c6-06c6853ee3c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan_019ef5c0-0cd0-7e84-8b15-64ae2efeed88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019ef5c0-0cd0-738f-a648-98b93130dc4b" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan_019ef5c0-0cd0-7e84-8b15-64ae2efeed88" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://golubcapitalbdc.com/role/SubsequentEventsScheduleofDeclaredDistributionstoHoldersDetails" xlink:type="simple" xlink:href="gbdc4-20250930.xsd#SubsequentEventsScheduleofDeclaredDistributionstoHoldersDetails"/>
  <link:definitionLink xlink:role="http://golubcapitalbdc.com/role/SubsequentEventsScheduleofDeclaredDistributionstoHoldersDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019ef5c0-0cd0-79ab-8873-273e999f1b9e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019ef5c0-0cd0-7744-a89b-58ff8f2b30a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019ef5c0-0cd0-79ab-8873-273e999f1b9e" xlink:to="loc_us-gaap_SubsequentEventTable_019ef5c0-0cd0-7744-a89b-58ff8f2b30a1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019ef5c0-0cd0-761a-8631-9be1dc66e537" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019ef5c0-0cd0-7744-a89b-58ff8f2b30a1" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019ef5c0-0cd0-761a-8631-9be1dc66e537" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-761a-8631-9be1dc66e537_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019ef5c0-0cd0-761a-8631-9be1dc66e537" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-761a-8631-9be1dc66e537_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-7b4e-87d7-5e4e11eb4714" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019ef5c0-0cd0-761a-8631-9be1dc66e537" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-7b4e-87d7-5e4e11eb4714" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019ef5c0-0cd0-73be-ae99-6c1c3a213731" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019ef5c0-0cd0-7b4e-87d7-5e4e11eb4714" xlink:to="loc_us-gaap_SubsequentEventMember_019ef5c0-0cd0-73be-ae99-6c1c3a213731" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_019ef5c0-0cd0-7a0f-a76a-b8efeea87a63" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019ef5c0-0cd0-7744-a89b-58ff8f2b30a1" xlink:to="loc_srt_StatementScenarioAxis_019ef5c0-0cd0-7a0f-a76a-b8efeea87a63" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0cd0-7a0f-a76a-b8efeea87a63_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_019ef5c0-0cd0-7a0f-a76a-b8efeea87a63" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0cd0-7a0f-a76a-b8efeea87a63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0cd0-7dea-9ecd-8867afcdd1d2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_019ef5c0-0cd0-7a0f-a76a-b8efeea87a63" xlink:to="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0cd0-7dea-9ecd-8867afcdd1d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_019ef5c0-0cd0-7cde-bf7f-11b6bef36548" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_019ef5c0-0cd0-7dea-9ecd-8867afcdd1d2" xlink:to="loc_srt_ScenarioForecastMember_019ef5c0-0cd0-7cde-bf7f-11b6bef36548" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentCompanyDistributionToShareholdersPerShare_019ef5c0-0cd0-74f7-b91e-e71811f99824" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyDistributionToShareholdersPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019ef5c0-0cd0-79ab-8873-273e999f1b9e" xlink:to="loc_us-gaap_InvestmentCompanyDistributionToShareholdersPerShare_019ef5c0-0cd0-74f7-b91e-e71811f99824" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-roles-2025.xsd#eedm"/>
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndEquitySecuritiesUnrealizedGainLoss" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtAndEquitySecuritiesUnrealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract" xlink:to="loc_us-gaap_DebtAndEquitySecuritiesUnrealizedGainLoss" xlink:type="arc" order="691"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossUnrealized" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossUnrealized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossUnrealized" xlink:type="arc" order="692"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossRealized" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossRealized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossRealized" xlink:type="arc" order="693"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossInvestmentIncomeOperating" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GrossInvestmentIncomeOperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract" xlink:to="loc_us-gaap_GrossInvestmentIncomeOperating" xlink:type="arc" order="694"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndEquitySecuritiesRealizedGainLoss" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtAndEquitySecuritiesRealizedGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementAbstract" xlink:to="loc_us-gaap_DebtAndEquitySecuritiesRealizedGainLoss" xlink:type="arc" order="695"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/cef/role/SecurityOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#SecurityOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/cef/role/SecurityOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DebtSecuritization2025Member_019ef5bf-fa75-723e-b576-980eb4678d56" xlink:href="gbdc4-20250930.xsd#gbdc4_DebtSecuritization2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_gbdc4_DebtSecuritization2025Member_019ef5bf-fa75-723e-b576-980eb4678d56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_PNCFacilityMember_019ef5bf-f8f5-70be-b878-1f092047a5fb" xlink:href="gbdc4-20250930.xsd#gbdc4_PNCFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_gbdc4_PNCFacilityMember_019ef5bf-f8f5-70be-b878-1f092047a5fb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_DBCreditFacilityMember_019ef5bf-f8f5-7b91-8bcd-0313e0e98cd1" xlink:href="gbdc4-20250930.xsd#gbdc4_DBCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_gbdc4_DBCreditFacilityMember_019ef5bf-f8f5-7b91-8bcd-0313e0e98cd1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_BNPCreditFacilityMember_019ef5bf-f8f5-7fdc-8dc3-9c52150b5154" xlink:href="gbdc4-20250930.xsd#gbdc4_BNPCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_gbdc4_BNPCreditFacilityMember_019ef5bf-f8f5-7fdc-8dc3-9c52150b5154" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AdvisorRevolverMember_019ef5bf-fc9d-7741-abf1-5931d0534efd" xlink:href="gbdc4-20250930.xsd#gbdc4_AdvisorRevolverMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_gbdc4_AdvisorRevolverMember_019ef5bf-fc9d-7741-abf1-5931d0534efd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gbdc4_AdviserRevolverMember_019ef5bf-f8f5-7a72-912a-d12ad4d491dd" xlink:href="gbdc4-20250930.xsd#gbdc4_AdviserRevolverMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain" xlink:to="loc_gbdc4_AdviserRevolverMember_019ef5bf-f8f5-7a72-912a-d12ad4d491dd" xlink:type="arc" order="6"/>
  </link:definitionLink>
</link:linkbase>
