Section 4. A license issued under this chapter on or after November first, nineteen hundred and forty-five, for operations as to which no certificate of public convenience and necessity under section seven is obtained within three years after the date when such license is issued by the licensing authority, which in a city shall mean the date of its approval by the mayor, shall become null and void upon the expiration of said three years. Such a license issued before said November first under the provisions of law applicable thereto or under this chapter, and remaining in force on said date, for operations as to which such a certificate is not obtained before the expiration of five years after said November first, shall become null and void upon the expiration of said five years. Each license so issued after July eighteenth, nineteen hundred and twenty-seven, for operations as to which such a certificate has been obtained prior to said November first, or is obtained within three years thereafter in the case of a license issued on or after said November first, or within five years thereafter in the case of a license issued before said date, shall remain in force and effect as to the routes and for the number of vehicles specified in such license or operated thereunder, notwithstanding any limitation contained therein as to the time it shall remain in effect, until revoked as hereinafter provided. After public notice and hearing, the licensing authority may, for good and sufficient reasons to be stated in the order of revocation, revoke in whole or in part such a license issued by such authority, but unless within thirty days after any such order of revocation, except an order made by the department or by the metropolitan district commission acting as such licensing authority, the licensee consents thereto in writing, such order shall not be valid until approved by the department after public notice and hearing.