4 hours at Dulles?

Bring a good book, or plan on what you are going to do in Munich.
As posted above, between the people-movers, security, and the extra time to get back in for an Int'l flight, you really do not have much time for anything outside the terminal.

If the airport is not too busy, and you get in on time, you could try for the Air & Space Museum, but REALLY make sure you have transportation waiting for you up and back. Even then, you will only get a cursory look at the museum (which may still be worth it).

Caveat -- if you have ANY thought that you might not get back (and through security) in time, DO NOT even try to go outside the terminal. It is not worth missing your flight, and IAD is just not reliably convenient to anywhere.

DaDOK