Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2025		Dec. 31, 2024		Mar. 31, 2024
Schedule of Investments [Line Items]
Cost	$ 389,132	[1]	$ 375,695	[2]
Fair Value	390,388	[1]	377,794	[2]	$ 377,794
Software
Schedule of Investments [Line Items]
Cost	184,773		177,698
Fair Value	185,186		178,395
Business Services
Schedule of Investments [Line Items]
Cost	99,460		102,830
Fair Value	99,794		103,117
Healthcare
Schedule of Investments [Line Items]
Cost	38,321		32,558
Fair Value	38,463		32,706
Financial Services
Schedule of Investments [Line Items]
Cost	23,401		21,538
Fair Value	23,434		21,570
Education
Schedule of Investments [Line Items]
Cost	15,190		14,646
Fair Value	15,190		14,741
Consumer Services
Schedule of Investments [Line Items]
Cost	13,390		11,870
Fair Value	13,398		11,933
Food & Beverage
Schedule of Investments [Line Items]
Cost	9,181		9,127
Fair Value	9,923		9,947
Packaging
Schedule of Investments [Line Items]
Cost	4,944		4,956
Fair Value	4,528		4,913
Investment Fund
Schedule of Investments [Line Items]
Cost	472		472
Fair Value	472		472
First lien
Schedule of Investments [Line Items]
Cost	372,740		359,930
Fair Value	373,863		361,945
Second lien
Schedule of Investments [Line Items]
Cost	9,902		9,337
Fair Value	10,004		9,417
Subordinated
Schedule of Investments [Line Items]
Cost	1,799		1,737
Fair Value	1,830		1,741
Structured finance obligations
Schedule of Investments [Line Items]
Cost	472		472
Fair Value	472		472
Equity and other
Schedule of Investments [Line Items]
Cost	4,219		4,219
Fair Value	$ 4,219		$ 4,219

[1] New Mountain Guardian IV Income Fund, L.L.C. (the "Company") generally acquires its investments in private transactions exempt from registration under the Securities Act of 1933, as amended (the "Securities Act"). These investments are generally subject to certain limitations on resale, and may be deemed to be "restricted securities" under the Securities Act.

[2] New Mountain Guardian IV Income Fund, L.L.C. (the "Company") generally acquires its investments in private transactions exempt from registration under the Securities Act of 1933, as amended (the "Securities Act"). These investments are generally subject to certain limitations on resale, and may be deemed to be "restricted securities" under the Securities Act.