If for any reason any tax payable under this Act, has escaped assessment or has been under assessed or assessed at a lower rate than the rate at which it is assessable, the assessing authority may at any time within four years from the expiry of the year to which the tax relates, proceed to assess or reassess the tax, as the case may be, to the best of its judgment after issuing a notice to the assessee concerned and after making such enquiry as it considers necessary. Provided that the tax shall be charges at the rate at which it would have been charged if such tax had not escaped assessment or, as the case may be, had not been under assessed or assessed at a rate lower than the rate at which it was assessable.