4. ¹[(1) Where, in the case of any sale, mortgagee or settlement, several instruments are employed for completing the transaction, the Principal instrument only shall be chargeable with the duty prescribed in Schedule I for the conveyance, mortgagee or settlement, and each of the other instruments shall be chargeable with a duty of ²[three hundred Taka] instead of the duty (if any) prescribed for it in that Schedule.] (2) The parties may determine for themselves which of the instruments so employed shall, for the purposes of sub-section (1), be deemed to be the principal instrument: Provided that the duty chargeable on the instrument so determined shall be the highest duty which would be chargeable in respect of any of the said instruments employed.