(iv)where any such debt or part of debt is written off as irrecoverable in the accounts of the previous year [(being a previous year relevant to the assessment year commencing on the 1st day of April, 1988, or any earlier assessment year)] [ Inserted by Act 4 of 1988, Section 11 (w.e.f. 1.4.1989).] and the [Assessing Officer] [ Substituted by Act 4 of 1988, Section 2, for " Income-tax Officer" (w.e.f. 1.4.1988).] is satisfied that such debt or part became a bad debt in any earlier previous year not falling beyond a period of four previous years immediately preceding the previous year in which such debt or part is written off, the provisions of sub-section (6) of section 155 shall apply;