If--
(a) the [Assessing Officer] has reason to believe that by reason of the omission or failure on the part of the assessee to make a return under section 5 for any assessment year or to disclose fully and truly all material facts necessary for his assessment for any assessment year, chargeable profits for that year have escaped assessment or have been under-assessed or assessed at too low a rate or have been made the subject of excessive relief under this Act, or
(b) notwithstanding that there has been no omission or failure as mentioned in clause (a) on the part of the assessee, the [Assessing Officer] has in consequence of information in his possession reason to believe that chargeable profits assessable for any assessment year have escaped assessment or have been under-assessed or assessed at too low a rate or have been the subject of excessive relief under this Act,
he may, in cases falling under clause (a) at any time, and in cases falling under clause (b) at any time within four years of the end of that assessment year, serve on the assessee a notice containing all or any of the requirements which may be included in a notice under section 5, and may proceed to assess or reassess the amount chargeable to surtax, and the provisions of this Act shall, so far as may be, apply as if the notice were a notice issued under that section.