Cover - shares	6 Months Ended
	Jun. 30, 2024	Aug. 13, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2024
Document Transition Report	false
Entity File Number	000-56437
Entity Registrant Name	New Mountain Guardian IV BDC, L.L.C.
Entity Address, Address Line One	1633 Broadway, 48th Floor
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10019
City Area Code	212
Local Phone Number	720-0300
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	88-1377220
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		44,882,706
Entity Central Index Key	0001925531
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q2