Document And Entity Information	6 Months Ended
	Sep. 30, 2020 shares
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Entity Registrant Name	BOSTON CAPITAL TAX CREDIT FUND IV LP
Title of 12(b) Security	None
No Trading Symbol Flag	true
Entity Central Index Key	0000913778
Current Fiscal Year End Date	--03-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding	0
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	true