Jack implies female, which makes me wonder how you know it has four connectors. I'm guessing you mean a TRS plug. (Or a TRRS plug, in this case.) (Actually, Wikipedia says that UK folks call these jack plugs, which is an oxymoron here.)

As Tony says, it's almost certainly a proprietary RS232 serial cable for some device that has limited space for a connector. Older TiVos had that setup, though I remember them as being TRS plugs, not TRRS.

This page would seem to refute the notion that it's for your PVR350, as there are only two connectors that look like they could accept that plug, and they're "Line In" and "Infrared". It also only mentions two in-the-box cables, and they don't match your description.