| Cover | Jan. 25, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K/A |
| Document Period End Date | Jan. 25, 2022 |
| Entity Registrant Name | X4 PHARMACEUTICALS, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-38295 |
| Entity Tax Identification Number | 27-3181608 |
| Entity Address, Address Line One | 61 North Beacon Street, |
| Entity Address, Address Line Two | 4th Floor |
| Entity Address, City or Town | Boston, |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02134 |
| City Area Code | 857 |
| Local Phone Number | 529-8300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 per share |
| Trading Symbol | XFOR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
| Entity Central Index Key | 0001501697 |
| Amendment Flag | false |
