The transfer time will depend on a number of things:

1. Are the connecting flights on the same ticket/airline? (I'm guessing it's BA)
2. Is the baggage checked through?
3. The class you are flying (LHR has a 'Fast Pass' system for premium passengers)
4. Whether the dreadful LHR security staff are feeling like they want to help that day.

If you can tick as many of these as possible, your transit will be smooth and fairly quick. In any event, 4 hours is plenty, even at Heathrow.

Can't help with the shopping and food options but I do know there are a lot of them.[msnsad]