Time limit for filing application u/s 54 of the Act

Can we file rectification application under section 154 at any time or we need to file this application within 4 years from the end of relevant assessment year in which the order sought to be revised was passed??

please clarify, as I understand.. this time limit is for AO and not for us as Assessee.. Because Section 154(8) starts with "without prejudice to the above".