Fair Value - Summary of Unobservable Inputs (Details) $ in Thousands	Dec. 31, 2024 USD ($)		Dec. 31, 2023 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 377,794	[1]	$ 86,613	[2]
First lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	361,945		83,830
Second lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	9,417		752
Subordinated
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,741		2,031
Equity and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	4,219
Level III
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	331,675		76,117
Level III | First lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	321,600
Level III | First lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 312,800		$ 59,470
Level III | First lien | Market & income approach | Low | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	8.5		8.5
Level III | First lien | Market & income approach | Low | Revenue Multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	3		8
Level III | First lien | Market & income approach | Low | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.068		0.072
Level III | First lien | Market & income approach | High | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	35		27
Level III | First lien | Market & income approach | High | Revenue Multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	13		0.10
Level III | First lien | Market & income approach | High | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.127		0.112
Level III | First lien | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	16.8		18.4
Level III | First lien | Market & income approach | Weighted Average | Revenue Multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	8.9		9
Level III | First lien | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.092		0.095
Level III | First lien | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 8,800		$ 15,895
Level III | Second lien
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	4,115
Level III | Second lien | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 4,115		$ 752
Level III | Second lien | Market & income approach | Low | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.099		12.9
Level III | Second lien | Market & income approach | High | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.102		12.9
Level III | Second lien | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.101		12.9
Level III | Subordinated
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,741		$ 0
Level III | Subordinated | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1,741
Level III | Subordinated | Market & income approach | Low | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	14.5
Level III | Subordinated | Market & income approach | Low | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.146
Level III | Subordinated | Market & income approach | High | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	15.5
Level III | Subordinated | Market & income approach | High | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.146
Level III | Subordinated | Market & income approach | Weighted Average | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	15
Level III | Subordinated | Market & income approach | Weighted Average | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.146
Level III | Equity and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 4,219
Level III | Equity and other | Market & income approach
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 802
Level III | Equity and other | Market & income approach | Low | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	12
Level III | Equity and other | Market & income approach | Low | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.135
Level III | Equity and other | Market & income approach | High | EBITDA multiple
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	18
Level III | Equity and other | Market & income approach | High | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	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	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	0.135
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.