Having spent way too much time in Heathrow - but usually Terminal 5 - my advice is to make your way to Terminal 4 immediately and get transport and security over with before thinking about food.
In T4 there is the inevitable Pret A Manger and T4 has a few casual restaurants and coffee places as well. You'll probably have a couple of hours to kill so you could do a restaurant but we usually just default to Pret A Manger and get as much as you feel - maybe a sandwich or salad and chips to split. And maybe coffee if you feel you need energy to make the last leg. Eating is a way to pass the time since time in the airport seems to slow to crawl while you're waiting for your gate announcement, but we're rarely hungry on the back of eating plane food and feeling out of time and place.
One tip: Pret a Manager had aluminum water bottles in T5 in May when we went through and they served us the whole trip as refillable bottles.
Paris will only start to opening up for dinner at 7:00 or 7:30 so getting food in Paris will not be an issue and staying up for a couple more hours may help you start to get on local time.
I'd say don't over think it. If you're really feeling hungry you'll have time to eat in LHR but there will be better food available in Paris when you get there so just do what you feel.
Don't fly hangry and enjoy Paris!
=Tod