40 ILCS 5/16-192

Should any change or error in the records result in any member, annuitant or beneficiary receiving from the system more or less than he or she would have been entitled to receive had the records been correct, the board shall correct such error, and, as far as practicable, shall adjust the payments in such a manner that the actuarial equivalent of the benefit to which such member, annuitant or beneficiary was entitled shall be paid; however, in no event shall the system be required to change the records of any member, annuitant or beneficiary if, at the time of discovery of the error, more than 4 fiscal years have elapsed since the fiscal year in which the error occurred.