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 may employ unregistered individuals to repair motor vehicles under the registrant’s immediate direction and control.