Section 4. The provisions of sections twenty-two and twenty-three of chapter one hundred and forty-nine relative to advertising during strikes shall cease to be operative when the board shall determine that the business of the employer, in respect to which the strike or other labor trouble occurred, is being carried on in the normal and usual manner and to the normal and usual extent. Upon the application of the employer, this question shall be determined by said board, but only after a full hearing at which all persons involved shall be entitled to be heard and represented by counsel. The board shall give at least three days’ notice of the hearing to the strikers and employees by publication in at least three daily newspapers published in the commonwealth, and by mailing a copy of said notice, postage prepaid, to the employers and to the accredited representatives of the strikers or workmen interested, when their addresses are known; and in every case the board shall make every reasonable and diligent effort to give notice to said strikers or interested workmen.