7 hours in London

Another place close by to Heathrow that is suggested for short stopovers is the National Trust property Osterley Park. This does require a mile walk from Osterley underground station to get to the house within the park, so good weather would make this more pleasant.

I'd suggest having a couple of plans depending on the length of time for immigration as well as weather on the day.

Just to put things in perspective:
Assume on time landing
at gate and deplane 9:20-9:30
walk to immigration- as much as 10-15 min.
wait and clear immigration & customs- from 20 min.-1 1/2 hours
find place to store luggage if needed - 15-30 min.
tube into London- appx. 45 min.-1 hour
That puts you London around noon. (and this assumes no delays on the tube/bus, etc.)
You need to be back at Heathrow and checking in /going through security at least 2-21/2 hours before your departure time- so much be back by 1:30- you have at least another 1 hour tube ride back to Heathrow....
You'd have MAYBE an optimistic 30 min. in London.

Bus to Windsor- 45 min.
Pre-booked car service to Windsor- 15-20 min.
You might have 2-2 1/2 hours in Windsor.

OR- you could get a day pass to one of the airline lounges and get a quick shower, a meal and some rest.

I'll add my vote to the "Not Enough Time" category.

Here's Heathrow's site. The place has 5 terminals, some rather old and boring, and at least one new and sparkly. Three are within walking distance of each other, two require a train (the Heathrow Express at the end of its route; shuffling between terminals is free.) I've killed time there by stashing my bags, picking out a decent restaurant in one of the terminals, lingering there, rummaging around the shops, etc.

My guess is that a long lunch will eat up most of your wait, since showing up for the 4pm flight at 1pm wouldn't be outrageous.

Looks like you've had sensible advice.

Thanks everyone for your helpful input. We've been to Windsor castle but haven't seen the actual town. It looks like there are several different buses - can anyone recommend the easiest route?
Alternately, any idea how much a cab/ private car hire would cost Heathrow to Windsor?

Which terminal are you arriving at?

Terminal 5

77 is the best.

Carol, I must certainly agree about not having enough time to see any of London. For context, it took my wife and I two hours to get from Heathrow to Central London on our recent visit and that was even with taking the Heathrow Express, which whisks you to Paddington in 15 minutes.