IAD is rather weak, with few windows facing the runways and limited restaurants and shops. The "temporary" Terminals C & D are dumps, crowded with tiny windows. The newer Terminals A & B are better with bigger windows and higher ceilings but views of the runway are blocked by C & D. The Saarinen main building is more impressive from the outside than the inside.
If you had a little more time, a trip to the Smithsonian's Udvar-Hazy Center would be interesting, located on the south end of the airport grounds. It is one of the largest hangers in the world, with several complete planes on display, including an SR
-71, a Concorde, the 707 prototype, the Enola Gay and the space shuttle Enterprise. Unfortunately, some of the displays are still incomplete and you cannot walk through any of the planes. They should park a 747-200 there and let people walk through the cargo hold or something. The center has an observation tower with a clear view but far away. If you have a camera with a long lens (200mm or more) you might be able to get some decent shots.
If you have a 4 hour layover, I presume you are connecting to an international flight, which would make getting over there a little tough. If you are on a domestic flight you might be able to squeeze in a couple of hours. The problem at Dulles is that depending on the time of day, going through security can take 5 minutes or 40 minutes and it is hard to know when it will be 40.