"To invoke equitable estoppel: (1) the party to be estopped must be apprised of the facts; (2) he must intend his conduct shall be acted upon, or must so act that the party asserting the estoppel had a right to believe it was so intended; (3) the other party must be ignorant of the true state of facts; (4) he must rely upon the conduct to his injury." (Barrett v. Stanislaus County Employees Retirement Assn. (1987) 189 Cal. App. 3d 1593, 1607 [234 Cal. Rptr. 900].)