At any time after four years from the date of the formation of a district, and from time to time thereafter, but not less than 120 days before a general district election, the board may, by resolution, increase the number of directors to serve thereon to 7, 9 or 11, and may designate the first additional members to serve on said enlarged board. The board shall determine the term of office of each of the new directors so appointed, but in no event shall such term designated by the board be for more than four years. The terms of office thus created shall be determined in such a manner as to keep as nearly equal as practicable the number of directors to be elected at each subsequent general district election. Upon the expiration of such term so designated by the board said directorship shall be filled at the next general district election and general district elections held thereafter for the election of officers of the district. The term of office of each director elected at a general district election is four years or until his successor qualifies and takes office.