(4)Irrespective of anything to the contrary contained in any of the provisions of Part C of this Chapter, where, in the case of an assessee, any amount of profits and gains of an undertaking or unit or enterprise or eligible business is claimed and allowed as a deduction under those provisions for any tax year,––(a)deduction in respect of, and to the extent of, such profits and gains shall not be allowed under any other provision of this Act for such tax year; and(b)shall in no case exceed the profits and gains of such undertaking or unit or enterprise or eligible business, as the case may be
