The term of office of all commissioners, except any ex officio member, shall be two years from the date of their election and until their successors are qualified; provided, however, that of the first members of the commission, one-half of the producers shall serve for one year, and one-half of the producers shall serve for two years, with the determination of term of each such member to be made by lot at the time of election. Terms of office of each commissioner shall be limited to four consecutive terms.