[ThePair]

If your TM was purchased used and not new, it’s certainly possible the fourth valve was damaged or leaking and prior owner fixed it with a plug and clamp. Is that one flexible hose (freshwater tank) or rigid (hot water heater)?

In any event, I’d just treat it like a valve for drainage and simply remove the clamp and plug for the winter.

If you purchased new, and it’s rigid, maybe TM decided to do this for the hot water tank, so that users didn’t accidentally burn out the heating element on a drained and empty tank. I’d still treat it as a winterizing drain and open it for the winter.