I agree with supermasterPhil:

A little tight to see Munich. Even if you were able to get into town in a timely fashion, you would only be able to see a very few places - standing around on the Marienplatz just to say you were there doesn't seem like a very effective use of your 4 hours.

I can recommend Freising - about 10km from the airport by road. It is a charming town with a decent main shopping street which you can easily walk end to end in an hour and a half. In fact, some of the best chocolates I have encountered can be found at Mittermeier Confisiorie (near the east end of Untere Hauptstrasse) on the main street.

cheers, MWAC