Section 13. If an electric railroad company, incorporated under this chapter, does not complete and open for use an extension within four years after the date of the authorization of the extension by the department, the power of the company to construct and operate the same shall cease, unless the department after public notice and a hearing, extends said time by a certificate stating that it deems due diligence has been exercised by the corporation, and that public necessity and convenience require the said extension of time.