Where to eat: LHR during layover or Paris 14th after 7pm (and 25 hours of travel)?

I’d just eat in one of the restaurants in Heathrow T4. It will be more comfortable than sitting in the terminal area. I think you’ll be exhausted by the time you arrive in Paris (I would be) and you will probably want an early bedtime. You can always pick up a snack in Paris from a grocery near your AirB&B.

You can check the menus for the HeathrowT4 restaurants online:
https://www.heathrow.com/at-the-airport/restaurants-a-z?type=restaurant&terminal=4

I try to avoid the airplane food when possible and grab something when I have a long layover, but it really depends on when you will be hungry and your plan for arrival night. 8-9pm is not that late for dinner in Europe generally, so I plan on having a restaurant dinner on arrival, getting on schedule for the rest of the trip. I would search what's near your airbnb if you think you'll be up for anything other than dropping into bed.

For my husband and me, it would be grabbing something to eat at LHR instead of (maybe) getting to our lodging at 7 pm. At that point, we would just want to walk around the neighborhood, unpack and get into bed early. In our experience, even eating at a bistro in Paris is at least a 1 1/2 hour experience. I doubt we would enjoy it thinking about being prone, instead.

I don't think you have to worry about getting gauged at LHR like you would at a US airport. If you look at the links posted by the other commenter, even the sit-down restaurant has "typical" prices, not airport ones. It's all going to come down to when your arriving flight actually gets in, whether your departing flight is delayed and how long it takes you to get to T4 from T3. If you have time to grab something, you'll probably be glad you did. If you end up sitting and waiting for most of that 4 hours, you'll probably wish you'd taken the time to sit down and eat a meal.
Enjoy your trip!

how much time do you have of that 4 hours? If one ticket you will want to follow the purple signs and get the airside bus to T4. If not on ticket you will be entering the UK and passport control (you will need an ETA in that case) and then make your own way to T4 and check in. You may not have a lot of time for lunch.

But assuming you stay airside, most eateries at Heathrow are airside. Very frustrating to me because I stay at the Crowne Plaza T4 as a non flying guest and food is very limited landside. The Heathrow app and website give your food choices on the map. Options at T4 are relatively limited even airside - a pub, a restaurant (Wild Olive - whatever that is), a Costa coffee and couple of take-aways including Pret a Manger.

It will have been a long day of traveling (about 25 total hours)

What kind of routing do you have that SEA-LHR-CDG takes 25 hours?

Your time at LHR will be 330AM to 730AM to your body, so depending what sort of meal service you have on the plane, you may or may not be ready for a ( main) meal.

Bring some snacks ( fruit/granola bars etc) so you can eat when the stomach signals it wants attention, in case you are somewhere a food vendor is not handy