4 hour stopover at Heathrow?
 

Worth going into London for or anywhere else for that matter?

If 4 hours is all you have in between flights (time landed to time taking off again), you won't have time. It takes about an hour to disembark and go thru immigration (assuming you are not EU). If you are talking about LHR, it's 15 minutes on HEX to get to Paddington (not including any wait time for HEX) and another 15-20 minutes to get to anything significant for tourists.
You have to allow for an hour to get back thru security at LHR. You also have to be at your gate by a certain minimum amount of time. With all this, your 4 hours are already gone without seeing anything!
You are better off just staying in the airport. In reality, you probably have only 2 hrs to wait. There's lots to do there.

Agree with above. You don't have time to leave. You'll have to stay at the airport. Go shopping or relax with a book or have a drink. Or go online and tell us how your flight was!

Shop at the duty free, relax and have some tea. No time to get in and out, seriously!

Thanks :)