Should the appellant or applicant commence construction or alteration within said two-year period, but should he fail to complete such construction or alteration within said four-year period, the Zoning Hearing Board may, upon 10 days' notice in writing, rescind or revoke the granted variance or special exception, or the issuance of the period, or permits, or the other action authorized to the appellant or applicant, if the Zoning Hearing Board finds that a good cause appears for the failure to complete the work within such four-year period, and if the Zoning Hearing Board further finds that conditions have so altered or changed in the interval since the granting of the variance, permit or action, that revocation or rescission of the action is justified.