Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2025		Dec. 31, 2024		Sep. 30, 2024
Schedule of Investments [Line Items]
Cost	$ 433,646	[1]	$ 375,695	[2]
Fair Value	432,413	[1]	377,794	[2]	$ 377,794
Software
Schedule of Investments [Line Items]
Cost	194,193		177,698
Fair Value	194,067		178,395
Business Services
Schedule of Investments [Line Items]
Cost	124,224		102,830
Fair Value	124,707		103,117
Financial Services
Schedule of Investments [Line Items]
Cost	35,702		21,538
Fair Value	35,852		21,570
Healthcare
Schedule of Investments [Line Items]
Cost	33,248		32,558
Fair Value	33,352		32,706
Education
Schedule of Investments [Line Items]
Cost	17,106		14,646
Fair Value	16,041		14,741
Consumer Services
Schedule of Investments [Line Items]
Cost	15,610		11,870
Fair Value	15,530		11,933
Business Products
Schedule of Investments [Line Items]
Cost	4,061
Fair Value	4,203
Packaging
Schedule of Investments [Line Items]
Cost	4,919		4,956
Fair Value	4,075		4,913
Food & Beverage
Schedule of Investments [Line Items]
Cost	3,352		9,127
Fair Value	3,351		9,947
Distribution & Logistics
Schedule of Investments [Line Items]
Cost	759
Fair Value	759
Investment Fund
Schedule of Investments [Line Items]
Cost	472		472
Fair Value	476		472
First lien
Schedule of Investments [Line Items]
Cost	409,894		359,930
Fair Value	408,665		361,945
Second lien
Schedule of Investments [Line Items]
Cost	17,073		9,337
Fair Value	17,004		9,417
Subordinated
Schedule of Investments [Line Items]
Cost	1,932		1,737
Fair Value	1,985		1,741
Structured finance obligations
Schedule of Investments [Line Items]
Cost	472		472
Fair Value	476		472
Equity and other
Schedule of Investments [Line Items]
Cost	4,275		4,219
Fair Value	$ 4,283		$ 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.