| Cover | Oct. 07, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 07, 2024 |
| Entity File Number | 814-01528 |
| Entity Registrant Name | New Mountain Guardian IV BDC, L.L.C. |
| Entity Central Index Key | 0001925531 |
| Entity Tax Identification Number | 88-1377220 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 1633 Broadway |
| Entity Address, Address Line Two | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Elected Not To Use the Extended Transition Period | false |
