What constitutes a prima facie case of age discrimination?

"In the context of the usual age discrimination case, a prima facie case of age discrimination arises when the employee shows that: (1) at the time of the adverse employment action, the employee was 40 years of age or older; (2) some adverse employment action was taken against the employee; (3) at the time of the adverse action the employee was satisfactorily performing his or her job; and (4) the employee was replaced in his or her position by a significantly younger person. [Citations.]" ( Muzquiz v. City of Emeryville (2000) 79 Cal.App.4th 1106, 1116, 94 Cal.Rptr.2d 579.)