v3.26.1
Cover - shares
9 Months Ended
Jun. 30, 2026
Aug. 06, 2026
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2026  
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 Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   81,864,005.909
Amendment Flag false  
Document Fiscal Year Focus 2026  
Document Fiscal Period Focus Q3  
Entity Central Index Key 0001901612  
Current Fiscal Year End Date --09-30