Section 4. Each certificate of registration issued by the deputy director shall bear a number, shall be valid until June first of the third year after issuance, may thereafter be renewed for a three year period upon application to the deputy director on a form provided by him, shall not be transferable, and may be exercised only by the person whose name appears on the certificate; provided, however, that the registered motor vehicle repair shop or registered motor vehicle glass repair shop may employ unregistered individuals to repair motor vehicles or motor vehicle glass, as the case may be under the registrant's immediate direction and control.