| Document and Entity Information | 3 Months Ended |
| Mar. 31, 2024 shares | |
| Document and Entity Information | |
| Document Type | 10-Q |
| Document Quarterly Report | true |
| Document Period End Date | Mar. 31, 2024 |
| Document Transition Report | false |
| Entity File Number | 033-99602-01 |
| Entity Registrant Name | BCTC IV ASSIGNOR CORP |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 04-3208532 |
| Entity Address, Address Line One | 225 Franklin Street |
| Entity Address, Address Line Two | 28th Floor |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02110 |
| City Area Code | 888 |
| Local Phone Number | 773-1487 |
| Title of 12(b) Security | None |
| Security Exchange Name | NONE |
| Entity Current Reporting Status | Yes |
| Entity Interactive Data Current | Yes |
| Entity Filer Category | Non-accelerated Filer |
| Entity Small Business | true |
| Entity Emerging Growth Company | false |
| Entity Shell Company | true |
| Entity Central Index Key | 0001003815 |
| Current Fiscal Year End Date | --12-31 |
| Document Fiscal Year Focus | 2024 |
| Document Fiscal Period Focus | Q1 |
| Amendment Flag | false |
| Entity Common Stock, Shares Outstanding | 1,000 |
| No Trading Symbol Flag | true |
| Condensed Balance Sheets - USD ($) | Mar. 31, 2024 | Dec. 31, 2023 |
| ASSETS | ||
| INVESTMENT IN OPERATING PARTNERSHIP (Note B) | $ 100 | $ 100 |
| ASSETS | 100 | 100 |
| LIABILITIES | ||
| Subscription payable | 100 | 100 |
| LIABILITIES | 100 | 100 |
| Stockholder's equity | ||
| Common stock - 1,000 shares authorized, issued and outstanding, $1 par value per share (Note A) | 1,000 | 1,000 |
| Less: subscription receivable | (1,000) | (1,000) |
| LIABILITIES AND STOCKHOLDER'S EQUITY | $ 100 | $ 100 |
| Condensed Balance Sheets (Parenthetical) - $ / shares | Mar. 31, 2024 | Dec. 31, 2023 |
| Condensed Balance Sheets | ||
| Common stock, par value (in dollars per share) | $ 1 | $ 1 |
| Common stock, shares authorized (in shares) | 1,000 | 1,000 |
| Common stock, shares issued (in shares) | 1,000 | 1,000 |
| Common stock, shares outstanding (in shares) | 1,000 | 1,000 |
| ORGANIZATION | 3 Months Ended |
| Mar. 31, 2024 | |
| ORGANIZATION | |
| ORGANIZATION | NOTE A - ORGANIZATION BCTC IV Assignor Corp., a Delaware corporation (the “Corporation”), was organized on October 12, 1993 to act as the assignor limited partner of BF Garden Tax Credit Fund IV L.P. (formerly known as Boston Capital Tax Credit Fund IV L.P.), a Delaware limited partnership (the “Limited Partnership”), and does not engage in any other business. Units of beneficial interest in the limited partnership interest of the Corporation in the Limited Partnership have been assigned by the Corporation by means of beneficial assignee certificates (“BACs”) to investors and investors are entitled to all the rights and economic benefits of a limited partner of the Limited Partnership, including rights to a percentage of the income, gains, losses, deductions, credits and distributions of the Limited Partnership. Effective as of December 15, 2020, BF Garden Companion Limited Partnership, a Massachusetts limited partnership formerly known as Boston Capital Companion Limited Partnership (“Companion”) replaced John P. Manning as the sole owner of the Corporation. Also effective as of December 15, 2020, BFBC Holdings GP, LLC, a Delaware limited liability company and an affiliate of Boston Financial Investment Management, LP, a Delaware limited partnership (“BFIM”), replaced Boston Capital Partners II Corporation as the general partner of Companion, and BFIM replaced Boston Capital Companion Holdings Limited Partnership as the sole limited partner of Companion. |
| INVESTMENT IN LIMITED PARTNERSHIP | 3 Months Ended |
| Mar. 31, 2024 | |
| INVESTMENT IN LIMITED PARTNERSHIP | |
| INVESTMENT IN LIMITED PARTNERSHIP | NOTE B - INVESTMENT IN LIMITED PARTNERSHIP On October 12, 1993, the Corporation was admitted as the assignor limited partner in the Limited Partnership. The Limited Partnership was formed to invest in real estate by acquiring, holding, and disposing of limited partnership interests in operating partnerships which will acquire, develop, rehabilitate, operate and own newly-constructed, existing or rehabilitated low-income apartment complexes. The Corporation has recorded its $100 investment at cost. |
| SUBSEQUENT EVENTS | 3 Months Ended |
| Mar. 31, 2024 | |
| SUBSEQUENT EVENTS | |
| SUBSEQUENT EVENTS | NOTE C - SUBSEQUENT EVENTS Events that occur after the balance sheet date but before the financial statements were available to be issued must be evaluated for recognition or disclosure. The effects of subsequent events that provide evidence about conditions that existed at the balance sheet date are recognized in the accompanying financial statements. Subsequent events, which provide evidence about conditions that existed after the balance sheet date, require disclosure in the accompanying notes. Management evaluated the activity of the Corporation through the date the financial statements were issued, and concluded that no subsequent events have occurred that would require recognition in the financial statements or disclosure in the notes to the financial statements. |
| INVESTMENT IN LIMITED PARTNERSHIP (Details) - USD ($) | 3 Months Ended | |
| Mar. 31, 2024 | Dec. 31, 2023 | |
| INVESTMENT IN LIMITED PARTNERSHIP | ||
| Limited partnership, formation date | Oct. 12, 1993 | |
| Investment at cost | $ 100 | $ 100 |
| Insider Trading Arrangements | 3 Months Ended |
| Mar. 31, 2024 | |
| Trading Arrangements, by Individual | |
| Rule 10b5-1 Arrangement Adopted | false |
| Non-Rule 10b5-1 Arrangement Adopted | false |
| Rule 10b5-1 Arrangement Terminated | false |
| Non-Rule 10b5-1 Arrangement Terminated | false |
