L.S.
Sounds like my son. :) Depending on the diaper situation (poo vs just pee), I found it a lot easier to sometimes change his diaper at the seat. Other than that, the bathroom would be your next bet. If you can manage a standing diaper change, that makes it easier. Always pack extra diapers and wipes -- like more than you think necessary. Better to have too many than too little.
It sounds like you're set with the toys and dvds. You might try adding new books, toys or dvds to your collection to make the moment even more interesting for him. My guy liked playing with aquadoodles and magnadoodles. He also liked eating and the more food I brought, the better he usually behaved. I always kept a small stash of airplane only type snacks for when or if I had to bring out the big guns and bribe him -- things like M&Ms, skittles, fruit loops, etc). To this day, my son thinks that Fruitloops and AppleJacks only exist at the airport. A cup of ice and those little straws still keeps my son entertained. Sometimes, it's the little things.
If he still naps, try running him around the airport terminal for a while before boarding. This seemed to wear my guy out -- especially if the flight overlapped with nap time.
All in all, you can only do so much. It sounds like you're fairly well prepared so I'm sure he'll be fine. It's only 4 hours and, in the scheme of things, this is not a long time for an adult so if anyone finds him annoying they'll get over it. Try and stay positive and I'm sure you'll have a good trip. Good luck!