4. An officer of Customs appointed under section 3 shall exercise such powers and discharge such duties as are conferred or imposed on him by or under this Act; and he shall also be competent to exercise all powers and discharge all duties conferred or imposed upon any officer subordinate to him: Provided that, notwithstanding anything contained in this Act or the rules, the Board may, by general or special order, impose such limitations or conditions on the exercise of such powers and discharge of such duties as it thinks fit.