18. (1) The Commissioner or the Appellate Assistant Commissioner may at any time within four years from the date of any order passed by him in appeal or in the case of the Commissioner in revision under section 17A and the Income-tax Officer may at any time within four years from the date of any assessment order passed by him on his own motion rectify any mistake apparent from the records of appeal, revision or assessment, as the case may be, and shall within the like period rectify any such mistake which has been brought to his notice by a person to whose business this Act applies: Provided that no such rectification shall be made having the effect of enhancing the liability of any person unless that person has been given a reasonable opportunity of being heard. (2) The provisions of sub-section (1) shall also apply in like manner to the rectification of mistakes by the Appellate Tribunal. (3) Where any such rectification has the effect of reducing the assessment, the Income-tax Officer shall make any refund which may be due to such assessee. (4) Where any such rectification has the effect of enhancing the assessment, the Income-tax Officer shall serve on the assessee a notice of demand in the prescribed form specifying the sum payable and such notice shall be deemed to be issued under section 29 of the Income-tax Act, 1922 as applied to the business profits tax under section 19 of this Act and the provisions of this Act shall apply accordingly.