Cover Page - shares	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Oct. 29, 2021
Cover [Abstract]
Document Type		10-Q
Document Quarterly Report		true
Document Period End Date	Sep. 30, 2021
Document Transition Report		false
Entity File Number		001-38295
Entity Registrant Name		X4 PHARMACEUTICALS, INC
Entity Incorporation, State or Country Code		DE
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
Title of 12(b) Security		Common Stock
Trading Symbol		XFOR
Security Exchange Name		NASDAQ
Entity Current Reporting Status		Yes
Entity Interactive Data Current		Yes
Entity Filer Category		Non-accelerated Filer
Entity Small Business		true
Entity Emerging Growth Company		true
Entity Ex Transition Period		true
Entity Shell Company		false
Entity Common Stock, Shares Outstanding			24,708,073
Amendment Flag		false
Document Fiscal Year Focus		2021
Document Fiscal Period Focus		Q3
Entity Central Index Key		0001501697
Current Fiscal Year End Date		--12-31