Fair Value - Summary of Unobservable Inputs (Details) $ in Thousands	Jun. 30, 2025 USD ($)		Dec. 31, 2024 USD ($)		Jun. 30, 2024 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 409,415	[1]	$ 377,794	[2]	$ 377,794
First lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	392,796				361,945
Second lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	9,971				9,417
Subordinated
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,908				1,741
Structured finance obligations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	474				472
Equity and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	4,266				4,219
Level III
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	384,865		331,675		331,675
Level III | First lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	368,246				321,600
Level III | First lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 356,553		$ 312,800
Level III | First lien | Market & income approach | Minimum | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.0		8.5
Level III | First lien | Market & income approach | Minimum | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	5.0		3
Level III | First lien | Market & income approach | Minimum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.064		0.068
Level III | First lien | Market & income approach | Maximum | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	36.0		35
Level III | First lien | Market & income approach | Maximum | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	13.0		13
Level III | First lien | Market & income approach | Maximum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.122		0.127
Level III | First lien | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	17.7		16.8
Level III | First lien | Market & income approach | Weighted Average | Revenue multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	10.2		8.9
Level III | First lien | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.087		0.092
Level III | First lien | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 11,694		$ 8,800
Level III | Second lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	9,971				4,115
Level III | Second lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 9,971		$ 4,115
Level III | Second lien | Market & income approach | Minimum | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	14
Level III | Second lien | Market & income approach | Minimum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	9		0.099
Level III | Second lien | Market & income approach | Maximum | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	18
Level III | Second lien | Market & income approach | Maximum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	14		0.102
Level III | Second lien | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	16
Level III | Second lien | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	9.3		0.101
Level III | Subordinated
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,908				1,741
Level III | Subordinated | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,908		$ 1,741
Level III | Subordinated | Market & income approach | Minimum | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	13.5		14.5
Level III | Subordinated | Market & income approach | Minimum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.143		0.146
Level III | Subordinated | Market & income approach | Maximum | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	15.5		15.5
Level III | Subordinated | Market & income approach | Maximum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.143		0.146
Level III | Subordinated | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	14.5		15
Level III | Subordinated | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.143		0.146
Level III | Structured finance obligations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 474				0
Level III | Structured finance obligations | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 474
Level III | Structured finance obligations | Market & income approach | Minimum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.113
Level III | Structured finance obligations | Market & income approach | Maximum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.113
Level III | Structured finance obligations | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.113
Level III | Equity and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 4,266				$ 4,219
Level III | Equity and other | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 4,265		$ 802
Level III | Equity and other | Market & income approach | Minimum | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	13.8		12
Level III | Equity and other | Market & income approach | Minimum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.9		0.135
Level III | Equity and other | Market & income approach | Maximum | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	18		18
Level III | Equity and other | Market & income approach | Maximum | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	13.4		0.135
Level III | Equity and other | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	17.4		16.3
Level III | Equity and other | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.118		0.135
Level III | Equity and other | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value			$ 3,417

[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.