California Code, Government Code - GOV § 1092

(a) Every contract made in violation of any of the provisions of Section 1090 may be avoided at the instance of any party except the officer interested therein. No such contract may be avoided because of the interest of an officer therein unless the contract is made in the official capacity of the officer, or by a board or body of which he or she is a member.

(b) An action under this section shall be commenced within four years after the plaintiff has discovered, or in the exercise of reasonable care should have discovered, a violation described in subdivision (a).