We had a long stopover -about 7 hours and were too tired/hung over to wander into London proper.
Booked Yotel for five hours... recharged our phones, slept like babies. It was perfect.
It can be a bit of a hike to get to Terminal 4 and maybe that is the only downside. The intra-terminal trains tend not to be synced very well and we had two 10 minute waits.
That is obviously not their fault, but something to consider if your stopover is shorter.More