I stayed at a Holiday Inn Express at Glasgow Airport last year, and found them quite comfortable for a good price, so I was happy to find out that Heathrow had one. It's connected to Terminal 4 and you get there by walking through a long tunnel from the terminal. The signage to get there is excellent. All you need to do is follow the signs for hotels, and specifically for the Holiday Inn Express. The service was good, the room was nice, and with a very comfortable bed. The hotel is connected to another IHG hotel, the Crown Royal Plaza, and everything was very open.
There were two restaurants, one a bit more casual, and one that was more formal. I opted for the casual and had the chicken makhani with a glass of wine. Both were very good. The hotel opens for breakfast at 5 am, but will let you order a breakfast bag if you have to leave earlier. I stayed one night and the cost was £98 (this was a non-cancellable rate, but I went with it as I had good travel insurance).