INVESTMENTS IN OPERATING PARTNERSHIPS - Current financial results available for the Operating Partnerships (Details) - USD ($)	6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Revenues
Rental	$ 10,846,001	$ 17,098,171
Interest and other	247,297	414,231
Operating Partnerships Revenues	11,093,298	17,512,402
Expenses
Interest	1,972,450	2,779,787
Depreciation and amortization	2,456,942	4,213,607
Operating expenses	7,446,917	12,566,968
Operating Partnerships Total Expenses	11,876,309	19,560,362
NET INCOME (LOSS)	(783,011)	(2,047,960)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(775,180)	(2,027,484)
Net income (loss) allocated to other partners	(7,831)	(20,476)
Series Twenty [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Twenty One [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Twenty Two [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Twenty Three [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Twenty Four [Member]
Revenues
Rental	0	422,317
Interest and other	0	19,531
Operating Partnerships Revenues	0	441,848
Expenses
Interest	0	36,469
Depreciation and amortization	0	118,903
Operating expenses	0	354,705
Operating Partnerships Total Expenses	0	510,077
NET INCOME (LOSS)	0	(68,229)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	(67,547)
Net income (loss) allocated to other partners	0	(682)
Series Twenty Five [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Twenty Six [Member]
Revenues
Rental	217,139	605,212
Interest and other	2,449	9,506
Operating Partnerships Revenues	219,588	614,718
Expenses
Interest	27,332	106,883
Depreciation and amortization	55,156	157,288
Operating expenses	222,991	569,196
Operating Partnerships Total Expenses	305,479	833,367
NET INCOME (LOSS)	(85,891)	(218,649)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(85,032)	(216,463)
Net income (loss) allocated to other partners	(859)	(2,186)
Series Twenty Seven [Member]
Revenues
Rental	0	211,201
Interest and other	0	125
Operating Partnerships Revenues	0	211,326
Expenses
Interest	0	6,529
Depreciation and amortization	0	67,956
Operating expenses	0	210,794
Operating Partnerships Total Expenses	0	285,279
NET INCOME (LOSS)	0	(73,953)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	(73,213)
Net income (loss) allocated to other partners	0	(740)
Series Twenty Eight [Member]
Revenues
Rental	0	263,303
Interest and other	0	8,837
Operating Partnerships Revenues	0	272,140
Expenses
Interest	0	64,522
Depreciation and amortization	0	69,046
Operating expenses	0	209,346
Operating Partnerships Total Expenses	0	342,914
NET INCOME (LOSS)	0	(70,774)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	(70,068)
Net income (loss) allocated to other partners	0	(706)
Series Twenty Nine [Member]
Revenues
Rental	417,945	464,228
Interest and other	12,890	43,983
Operating Partnerships Revenues	430,835	508,211
Expenses
Interest	67,252	79,653
Depreciation and amortization	74,363	92,325
Operating expenses	369,978	421,661
Operating Partnerships Total Expenses	511,593	593,639
NET INCOME (LOSS)	(80,758)	(85,428)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(79,950)	(84,574)
Net income (loss) allocated to other partners	(808)	(854)
Series Thirty [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Thirty One [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Thirty Two [Member]
Revenues
Rental	585,243	761,862
Interest and other	9,746	13,701
Operating Partnerships Revenues	594,989	775,563
Expenses
Interest	88,374	125,371
Depreciation and amortization	174,189	219,045
Operating expenses	501,400	613,804
Operating Partnerships Total Expenses	763,963	958,220
NET INCOME (LOSS)	(168,974)	(182,657)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(167,284)	(180,830)
Net income (loss) allocated to other partners	(1,690)	(1,827)
Series Thirty Three [Member]
Revenues
Rental	448,911	416,301
Interest and other	15,830	13,138
Operating Partnerships Revenues	464,741	429,439
Expenses
Interest	74,656	79,953
Depreciation and amortization	96,068	100,726
Operating expenses	345,562	307,186
Operating Partnerships Total Expenses	516,286	487,865
NET INCOME (LOSS)	(51,545)	(58,426)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(51,030)	(57,842)
Net income (loss) allocated to other partners	(515)	(584)
Series Thirty Four [Member]
Revenues
Rental	326,724	408,120
Interest and other	11,341	15,105
Operating Partnerships Revenues	338,065	423,225
Expenses
Interest	49,965	54,704
Depreciation and amortization	76,498	106,772
Operating expenses	280,893	362,170
Operating Partnerships Total Expenses	407,356	523,646
NET INCOME (LOSS)	(69,291)	(100,421)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(68,598)	(99,417)
Net income (loss) allocated to other partners	(693)	(1,004)
Series Thirty Five [Member]
Revenues
Rental	0	404,488
Interest and other	0	2,157
Operating Partnerships Revenues	0	406,645
Expenses
Interest	0	58,450
Depreciation and amortization	0	131,653
Operating expenses	0	218,081
Operating Partnerships Total Expenses	0	408,184
NET INCOME (LOSS)	0	(1,539)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	(1,524)
Net income (loss) allocated to other partners	0	(15)
Series Thirty Six [Member]
Revenues
Rental	0	88,773
Interest and other	0	810
Operating Partnerships Revenues	0	89,583
Expenses
Interest	0	21,196
Depreciation and amortization	0	27,363
Operating expenses	0	85,948
Operating Partnerships Total Expenses	0	134,507
NET INCOME (LOSS)	0	(44,924)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	(44,475)
Net income (loss) allocated to other partners	0	(449)
Series Thirty Seven [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Thirty Eight [Member]
Revenues
Rental	0	115,212
Interest and other	0	887
Operating Partnerships Revenues	0	116,099
Expenses
Interest	0	13,168
Depreciation and amortization	0	27,635
Operating expenses	0	99,031
Operating Partnerships Total Expenses	0	139,834
NET INCOME (LOSS)	0	(23,735)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	(23,498)
Net income (loss) allocated to other partners	0	(237)
Series Thirty Nine [Member]
Revenues
Rental	0	0
Interest and other	0	0
Operating Partnerships Revenues	0	0
Expenses
Interest	0	0
Depreciation and amortization	0	0
Operating expenses	0	0
Operating Partnerships Total Expenses	0	0
NET INCOME (LOSS)	0	0
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	0
Net income (loss) allocated to other partners	0	0
Series Forty [Member]
Revenues
Rental	0	486,308
Interest and other	0	9,251
Operating Partnerships Revenues	0	495,559
Expenses
Interest	0	80,247
Depreciation and amortization	0	122,649
Operating expenses	0	362,884
Operating Partnerships Total Expenses	0	565,780
NET INCOME (LOSS)	0	(70,221)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	0	(69,519)
Net income (loss) allocated to other partners	0	(702)
Series Forty One [Member]
Revenues
Rental	1,524,961	1,780,047
Interest and other	38,570	31,950
Operating Partnerships Revenues	1,563,531	1,811,997
Expenses
Interest	275,662	283,344
Depreciation and amortization	280,652	368,462
Operating expenses	1,014,090	1,294,393
Operating Partnerships Total Expenses	1,570,404	1,946,199
NET INCOME (LOSS)	(6,873)	(134,202)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(6,804)	(132,860)
Net income (loss) allocated to other partners	(69)	(1,342)
Series Forty Two [Member]
Revenues
Rental	964,900	1,131,169
Interest and other	44,353	42,952
Operating Partnerships Revenues	1,009,253	1,174,121
Expenses
Interest	187,840	217,395
Depreciation and amortization	244,575	323,583
Operating expenses	781,356	851,969
Operating Partnerships Total Expenses	1,213,771	1,392,947
NET INCOME (LOSS)	(204,518)	(218,826)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(202,473)	(216,638)
Net income (loss) allocated to other partners	(2,045)	(2,188)
Series Forty Three [Member]
Revenues
Rental	430,480	1,312,145
Interest and other	13,457	36,262
Operating Partnerships Revenues	443,937	1,348,407
Expenses
Interest	31,491	113,547
Depreciation and amortization	170,133	372,693
Operating expenses	396,077	1,068,124
Operating Partnerships Total Expenses	597,701	1,554,364
NET INCOME (LOSS)	(153,764)	(205,957)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(152,226)	(203,897)
Net income (loss) allocated to other partners	(1,538)	(2,060)
Series Forty Four [Member]
Revenues
Rental	1,817,906	2,449,364
Interest and other	20,511	51,200
Operating Partnerships Revenues	1,838,417	2,500,564
Expenses
Interest	435,314	553,571
Depreciation and amortization	339,618	513,484
Operating expenses	786,743	1,447,637
Operating Partnerships Total Expenses	1,561,675	2,514,692
NET INCOME (LOSS)	276,742	(14,128)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	273,975	(13,987)
Net income (loss) allocated to other partners	2,767	(141)
Series Forty Five [Member]
Revenues
Rental	1,786,650	3,290,532
Interest and other	24,785	76,870
Operating Partnerships Revenues	1,811,435	3,367,402
Expenses
Interest	270,649	404,349
Depreciation and amortization	447,266	826,542
Operating expenses	1,302,011	2,560,676
Operating Partnerships Total Expenses	2,019,926	3,791,567
NET INCOME (LOSS)	(208,491)	(424,165)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(206,406)	(419,923)
Net income (loss) allocated to other partners	(2,085)	(4,242)
Series Forty Six [Member]
Revenues
Rental	2,325,142	2,487,589
Interest and other	53,365	37,966
Operating Partnerships Revenues	2,378,507	2,525,555
Expenses
Interest	463,915	480,436
Depreciation and amortization	498,424	567,482
Operating expenses	1,445,816	1,529,363
Operating Partnerships Total Expenses	2,408,155	2,577,281
NET INCOME (LOSS)	(29,648)	(51,726)
Net income (loss) allocated to Boston Capital Tax Credit Fund IV L.P.	(29,352)	(51,209)
Net income (loss) allocated to other partners	$ (296)	$ (517)