4 hours will be pretty tight for doing that, and almost impossible via public transport - DART is pretty useless in my experience. Frequency is something like once hourly.

If you arrived in DFW on time, you could conceivably do this via Uber or taxi, but traffic can be awful and you wouldn't likely have much time to do anything other than drive by Dealey plaza. The Sixth Floor Museum is worth seeing but I just don't think that you have time for that.

The airport itself is out in suburbia, with little worth seeing (in my opinion, and I lived in Texas for quite a while) - myself, I'd just stick to the lounge, or without lounge access, I'd park myself at once of the restaurants or bars in the airport. Terminal D, as I recall, has some pretty good choices, and once airside, you can move around easily between terminals.