If you're not eligible for lounge access four hours is plenty of time to clear immigration, take the Heathrow Express to T4 (free), go through the walkway to the Hilton and hang out in the lounge there. Nicer and less crowded than most airline lounges (I've been to 6 or 8 at LHR). As flight time approaches, head for T5.

In fact, it's enough for a short trip to the local countryside if you want. The local bus to the A4 roadway along the north side of the airport is free, too.

A bit tight for getting into London, though, unless you just want to say you did.