Legal Question in Investment Law in California

I'm doubtful that there is a "statute of limitations." Statutes of limitations normally limit the time a potential plaintiff has to bring a lawsuit or prosecution. Here, there is no court action involved. This is an accounting procedure under a contract, not a court action. There's still a question as to whether the employer is right, of course. I'd suggest starting with a careful review of the vesting provisions of the particular 401(k) plan, followed with an "informed discussion" with the employer's 401(k) administrator.