v3.26.1
Cover - USD ($)
 12 Months Ended
Sep. 30, 2025
Nov. 21, 2025
Cover [Abstract]    
Document Type 10-K/A  
Document Annual Report true  
Document Period End Date Sep. 30, 2025  
Current Fiscal Year End Date --09-30  
Document Transition Report false  
Entity File Number 814-01504  
Entity Registrant Name Golub Capital BDC 4, Inc.  
Entity Incorporation, State or Country Code MD  
Entity Tax Identification Number 88-1608711  
Entity Address, Address Line One 200 Park Avenue  
Entity Address, Address Line Two 25th Floor  
Entity Address, City or Town New York  
Entity Address, State or Province NY  
Entity Address, Postal Zip Code 10166  
City Area Code 212  
Local Phone Number 750-6060  
Entity Well-known Seasoned Issuer No  
Entity Voluntary Filers No  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
ICFR Auditor Attestation Flag true  
Document Financial Statement Error Correction [Flag] false  
Entity Shell Company false  
Entity Public Float $ 0  
Entity Common Stock, Shares Outstanding   71,496,901.607
Amendment Flag true  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus FY  
Entity Central Index Key 0001901612  
Amendment Description Golub Capital BDC 4, Inc. (“GBDC 4” or “the Company”) is filing this Amendment No. 1 on Form 10-K/A (the “Form 10-K/A”) to its Annual Report on Form 10-K filed with the Securities and Exchange Commission (the “SEC”) on November 21, 2025 (the “Original Filing”) for the purpose of correcting the inadvertent omission of a subsection in the Ernst & Young LLP’s Report of Independent Registered Public Accounting Firm (“Auditor’s Report”) included in Item 8. The auditor’s opinion on the Company’s consolidated financial statements remained unchanged.The error that has been corrected is the omission of a subsection from the Auditor’s Report. This subsection is included in the Auditor’s Report on Form 10-K/A. The Auditor’s Report was included in the Original Filing. Except for the aforementioned correction to the Auditor’s Report, this Amendment does not amend, modify or update the Original Filing in any respect. Information included in this Amendment is stated as of September 30, 2025 and does not reflect events that have occurred subsequent to the filing of the Original Filing and, accordingly, this Amendment should be read in conjunction with our Original Filing made with the SEC.This Annual Report on Form 10-K/A consists of a cover page, this explanatory note, Item 8 (as amended) of the 2025 Annual Report on Form 10-K (including our consolidated financial statements and the corrected Auditor’s Report), Item 15 of the 2025 Annual Report on Form 10-K, the signature page and the required certifications of GBDC 4’s chief executive officer and the chief financial officer.