Where reopening notice for AY 2015-16 was issued on 31-3-2021 after obtaining approval of Principal Commissioner, since four years from the end of assessment year fell between 20-3-2020 and 31-3-2021, proceedings were governed by the old regime and section 151(2) was applicable, under which satisfaction of Joint Commissioner was required; approval granted by Principal Commissioner was therefore not valid sanction, and consequential notice under section 148 and assessment order passed under section 147 were quashed. (AY. 2015-16)