What you can achieve in that 4h50m depends on a large number of variables including:
Whether your flight arrives early or late;
Whether you have EU passports or not;
If you do, whether they can be read by the biometric passport readers or not;
If you don't, how long is the queue at immigration fast track;

If you arrive early or on time, and you have EU biometric passports, or non-EU passports and there's little or no queue at fast track immigration, then realistically you will have 3h to 3h30m max to play with.

That will be enough time to get you to Paddington and back on the HEX with a couple of hours to wander around and a quick lunch.