Marmar, just remembered there is a Hilton hotel attached by a walkway to Terminal 4.
It apparently has several restaurants. We have never been there.
If you are not in Terminal 4, it is not just a casual walk to get there from another terminal, trust me on this.
There are other hotels not too far away from the terminal and I suppose you could take a cab there and back.
The tube does go to the terminals and I suppose you could try to navigate that.
Would make sure I was checked in and had a boarding pass before trying to go anywhere (getting on a plane can take forever in Heathrow). Last time we were there had to wait before we could even check our bags.
We have spent far too much time in Heathrow, usually Terminal 4 (British Air), and it is not a flier friendly place.
But have a great trip.