The penalty, and the way out of it
The Department of Labor assesses a penalty for each day a return is late, indexed annually and currently running well above two thousand dollars a day. There is no cap. The IRS can assess its own separate per-day penalty on top. Because it accrues from the original due date, a plan that stopped filing four years ago is not looking at four penalties — it is looking at roughly fifteen hundred days of them.
The Delinquent Filer Voluntary Compliance Program exists precisely for this. File the missing returns voluntarily, before the Department contacts you, and the penalty is reduced to a fixed amount per return with an annual cap that is a small fraction of the exposure. The programme is only available while the failure is genuinely voluntary. Once a letter arrives, it is closed.
If you suspect a return has been missed, the arithmetic strongly favours dealing with it this month rather than next. We can establish what was due, prepare the filings and take them through the programme.