Section 4. Any optometric service corporation may enter into contracts with its subscribers and with participating optometrists for the rendering of optometric service to the subscribers. A contract with a subscriber may provide for the optometric care of any dependents of the subscriber named therein. The extent of optometric services to be provided and the form of agreement with participating optometrists shall at all times be subject to the written approval of the commissioner. The fees to be paid to participating optometrists for their services to the subscribers or to insured dependents and the rates charged by such corporation to the subscribers shall at all times be subject to a public hearing as provided by section two of chapter thirty A. Such fees shall be lower than the fees charged by participating optometrists to their non-subscriber patients. The form of subscription certificate, the rates charged by such corporation to the subscribers and the fees at which participating optometrists are compensated for their services to the subscribers or to covered dependents, shall at all times be subject to the written approval of the commissioner. Acquisition costs in connection with the solicitation of subscribers and costs of administration shall at all times be limited to such amounts as the commissioner shall approve.