Cover - USD ($)	12 Months Ended
	Dec. 31, 2024	Mar. 05, 2025	Jun. 30, 2024
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2024
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	814-01639
Entity Registrant Name	New Mountain Guardian IV Income Fund, 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	92-0964074
Title of 12(g) Security	Units of Limited Liability Company Interests
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	true
Entity Ex Transition Period	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		33,306,325
Entity Public Float			$ 0
Entity Central Index Key	0001976719
Amendment Flag	false
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	FY
Year Of Inception