65 ILCS 5/4-3-4

(a) Except as otherwise provided in this article, the mayor and commissioners elected under Section 4-3-1 shall hold their respective offices for the term of 4 years and until their successors are elected and have qualified. (b) If a vacancy occurs in any of these offices, the remaining members of the council, within 30 days after the vacancy occurs, shall appoint a person to fill the vacancy for the balance of the unexpired term or until the vacancy is filled by interim election under Section 3.1-10-50 and until the successor is elected and has qualified.