If a variance or special exception is granted, or the issuance of a permit is approved, or other action authorized, the necessary permit shall be secured and the authorized action begun within two years after the date when the variance or special exception is finally granted, or the issuance of a permit is finally approved, or the other action authorized, and the building or alteration, as the case may be, shall be completed within four years of said date. For good cause, the Board may, upon application in writing, stating the reasons therefor, extend either the two-year or four-year period.