INVESTMENTS IN OPERATING PARTNERSHIPS - Current financial results available for the Operating Partnerships (Details) - USD ($)	6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Revenues
Rental	$ 17,098,171	$ 22,343,323
Interest and other	414,231	663,683
Operating Partnerships Revenues	17,512,402	23,007,006
Expenses
Interest	2,779,787	3,644,356
Depreciation and amortization	4,213,607	5,775,862
Operating expenses	12,566,968	16,122,850
Operating Partnerships Total Expenses	19,560,362	25,543,068
NET LOSS	(2,047,960)	(2,536,062)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(2,027,484)	(2,510,699)
Net loss allocated to other Partners	(20,476)	(25,363)
Series Twenty [Member]
Revenues
Rental		139,480
Interest and other		9,921
Operating Partnerships Revenues		149,401
Expenses
Interest		11,732
Depreciation and amortization		28,416
Operating expenses		124,517
Operating Partnerships Total Expenses		164,665
NET LOSS		(15,264)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.		(15,111)
Net loss allocated to other Partners		(153)
Series Twenty Two [Member]
Revenues
Rental		255,007
Interest and other		6,424
Operating Partnerships Revenues		261,431
Expenses
Interest		21,753
Depreciation and amortization		54,539
Operating expenses		215,777
Operating Partnerships Total Expenses		292,069
NET LOSS		(30,638)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.		(30,332)
Net loss allocated to other Partners		(306)
Series Twenty Three [Member]
Revenues
Rental		204,371
Interest and other		8,421
Operating Partnerships Revenues		212,792
Expenses
Interest		14,889
Depreciation and amortization		45,095
Operating expenses		169,287
Operating Partnerships Total Expenses		229,271
NET LOSS		(16,479)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.		(16,314)
Net loss allocated to other Partners		(165)
Series Twenty Four [Member]
Revenues
Rental	422,317	428,402
Interest and other	19,531	13,733
Operating Partnerships Revenues	441,848	442,135
Expenses
Interest	36,469	34,789
Depreciation and amortization	118,903	121,938
Operating expenses	354,705	342,441
Operating Partnerships Total Expenses	510,077	499,168
NET LOSS	(68,229)	(57,033)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(67,547)	(56,463)
Net loss allocated to other Partners	(682)	(570)
Series Twenty Six [Member]
Revenues
Rental	605,212	637,353
Interest and other	9,506	7,655
Operating Partnerships Revenues	614,718	645,008
Expenses
Interest	106,883	111,673
Depreciation and amortization	157,288	155,406
Operating expenses	569,196	536,826
Operating Partnerships Total Expenses	833,367	803,905
NET LOSS	(218,649)	(158,897)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(216,463)	(157,308)
Net loss allocated to other Partners	(2,186)	(1,589)
Series Twenty Seven [Member]
Revenues
Rental	211,201	330,303
Interest and other	125	6,495
Operating Partnerships Revenues	211,326	336,798
Expenses
Interest	6,529	16,620
Depreciation and amortization	67,956	88,925
Operating expenses	210,794	275,358
Operating Partnerships Total Expenses	285,279	380,903
NET LOSS	(73,953)	(44,105)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(73,213)	(43,664)
Net loss allocated to other Partners	(740)	(441)
Series Twenty Eight [Member]
Revenues
Rental	263,303	411,179
Interest and other	8,837	22,570
Operating Partnerships Revenues	272,140	433,749
Expenses
Interest	64,522	66,567
Depreciation and amortization	69,046	91,762
Operating expenses	209,346	340,875
Operating Partnerships Total Expenses	342,914	499,204
NET LOSS	(70,774)	(65,455)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(70,068)	(64,800)
Net loss allocated to other Partners	(706)	(655)
Series Twenty Nine [Member]
Revenues
Rental	464,228	543,919
Interest and other	43,983	21,495
Operating Partnerships Revenues	508,211	565,414
Expenses
Interest	79,653	106,247
Depreciation and amortization	92,325	114,458
Operating expenses	421,661	459,302
Operating Partnerships Total Expenses	593,639	680,007
NET LOSS	(85,428)	(114,593)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(84,574)	(113,447)
Net loss allocated to other Partners	(854)	(1,146)
Series Thirty [Member]
Revenues
Rental		322,299
Interest and other		7,537
Operating Partnerships Revenues		329,836
Expenses
Interest		34,088
Depreciation and amortization		76,907
Operating expenses		261,993
Operating Partnerships Total Expenses		372,988
NET LOSS		(43,152)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.		(42,720)
Net loss allocated to other Partners		(432)
Series Thirty One [Member]
Revenues
Rental		563,108
Interest and other		21,066
Operating Partnerships Revenues		584,174
Expenses
Interest		41,564
Depreciation and amortization		209,468
Operating expenses		563,223
Operating Partnerships Total Expenses		814,255
NET LOSS		(230,081)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.		(227,780)
Net loss allocated to other Partners		(2,301)
Series Thirty Two [Member]
Revenues
Rental	761,862	770,495
Interest and other	13,701	11,472
Operating Partnerships Revenues	775,563	781,967
Expenses
Interest	125,371	128,444
Depreciation and amortization	219,045	219,165
Operating expenses	613,804	641,294
Operating Partnerships Total Expenses	958,220	988,903
NET LOSS	(182,657)	(206,936)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(180,830)	(204,867)
Net loss allocated to other Partners	(1,827)	(2,069)
Series Thirty Three [Member]
Revenues
Rental	416,301	632,934
Interest and other	13,138	23,019
Operating Partnerships Revenues	429,439	655,953
Expenses
Interest	79,953	101,675
Depreciation and amortization	100,726	139,535
Operating expenses	307,186	465,670
Operating Partnerships Total Expenses	487,865	706,880
NET LOSS	(58,426)	(50,927)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(57,842)	(50,418)
Net loss allocated to other Partners	(584)	(509)
Series Thirty Four [Member]
Revenues
Rental	408,120	495,011
Interest and other	15,105	25,201
Operating Partnerships Revenues	423,225	520,212
Expenses
Interest	54,704	57,916
Depreciation and amortization	106,772	137,095
Operating expenses	362,170	370,981
Operating Partnerships Total Expenses	523,646	565,992
NET LOSS	(100,421)	(45,780)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(99,417)	(45,322)
Net loss allocated to other Partners	(1,004)	(458)
Series Thirty Five [Member]
Revenues
Rental	404,488	504,934
Interest and other	2,157	28,304
Operating Partnerships Revenues	406,645	533,238
Expenses
Interest	58,450	83,244
Depreciation and amortization	131,653	161,156
Operating expenses	218,081	303,777
Operating Partnerships Total Expenses	408,184	548,177
NET LOSS	(1,539)	(14,939)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(1,524)	(14,790)
Net loss allocated to other Partners	(15)	(149)
Series Thirty Six [Member]
Revenues
Rental	88,773	284,874
Interest and other	810	5,290
Operating Partnerships Revenues	89,583	290,164
Expenses
Interest	21,196	52,992
Depreciation and amortization	27,363	70,217
Operating expenses	85,948	208,729
Operating Partnerships Total Expenses	134,507	331,938
NET LOSS	(44,924)	(41,774)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(44,475)	(41,356)
Net loss allocated to other Partners	(449)	(418)
Series Thirty Eight [Member]
Revenues
Rental	115,212	810,726
Interest and other	887	16,373
Operating Partnerships Revenues	116,099	827,099
Expenses
Interest	13,168	111,239
Depreciation and amortization	27,635	155,514
Operating expenses	99,031	564,808
Operating Partnerships Total Expenses	139,834	831,561
NET LOSS	(23,735)	(4,462)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(23,498)	(4,417)
Net loss allocated to other Partners	(237)	(45)
Series Forty [Member]
Revenues
Rental	486,308	1,017,456
Interest and other	9,251	26,894
Operating Partnerships Revenues	495,559	1,044,350
Expenses
Interest	80,247	189,214
Depreciation and amortization	122,649	276,150
Operating expenses	362,884	794,063
Operating Partnerships Total Expenses	565,780	1,259,427
NET LOSS	(70,221)	(215,077)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(69,519)	(212,926)
Net loss allocated to other Partners	(702)	(2,151)
Series Forty One [Member]
Revenues
Rental	1,780,047	1,815,549
Interest and other	31,950	29,661
Operating Partnerships Revenues	1,811,997	1,845,210
Expenses
Interest	283,344	340,027
Depreciation and amortization	368,462	384,753
Operating expenses	1,294,393	1,223,663
Operating Partnerships Total Expenses	1,946,199	1,948,443
NET LOSS	(134,202)	(103,233)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(132,860)	(102,201)
Net loss allocated to other Partners	(1,342)	(1,032)
Series Forty Two [Member]
Revenues
Rental	1,131,169	1,196,029
Interest and other	42,952	47,542
Operating Partnerships Revenues	1,174,121	1,243,571
Expenses
Interest	217,395	242,115
Depreciation and amortization	323,583	362,541
Operating expenses	851,969	876,593
Operating Partnerships Total Expenses	1,392,947	1,481,249
NET LOSS	(218,826)	(237,678)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(216,638)	(235,301)
Net loss allocated to other Partners	(2,188)	(2,377)
Series Forty Three [Member]
Revenues
Rental	1,312,145	2,093,498
Interest and other	36,262	90,363
Operating Partnerships Revenues	1,348,407	2,183,861
Expenses
Interest	113,547	256,666
Depreciation and amortization	372,693	695,187
Operating expenses	1,068,124	1,576,475
Operating Partnerships Total Expenses	1,554,364	2,528,328
NET LOSS	(205,957)	(344,467)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(203,897)	(341,022)
Net loss allocated to other Partners	(2,060)	(3,445)
Series Forty Four [Member]
Revenues
Rental	2,449,364	2,981,014
Interest and other	51,200	81,854
Operating Partnerships Revenues	2,500,564	3,062,868
Expenses
Interest	553,571	695,577
Depreciation and amortization	513,484	676,174
Operating expenses	1,447,637	1,711,083
Operating Partnerships Total Expenses	2,514,692	3,082,834
NET LOSS	(14,128)	(19,966)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(13,987)	(19,766)
Net loss allocated to other Partners	(141)	(200)
Series Forty Five [Member]
Revenues
Rental	3,290,532	3,545,117
Interest and other	76,870	80,792
Operating Partnerships Revenues	3,367,402	3,625,909
Expenses
Interest	404,349	455,418
Depreciation and amortization	826,542	944,459
Operating expenses	2,560,676	2,613,391
Operating Partnerships Total Expenses	3,791,567	4,013,268
NET LOSS	(424,165)	(387,359)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(419,923)	(383,485)
Net loss allocated to other Partners	(4,242)	(3,874)
Series Forty Six [Member]
Revenues
Rental	2,487,589	2,360,265
Interest and other	37,966	71,601
Operating Partnerships Revenues	2,525,555	2,431,866
Expenses
Interest	480,436	469,907
Depreciation and amortization	567,482	567,002
Operating expenses	1,529,363	1,482,724
Operating Partnerships Total Expenses	2,577,281	2,519,633
NET LOSS	(51,726)	(87,767)
Net loss allocated to Boston Capital Tax Credit Fund IV L.P.	(51,209)	(86,889)
Net loss allocated to other Partners	$ (517)	$ (878)