I think the one thing to do in Salzburg with 4-5 hours is just to enjoy the buildings and the vibe. I love Salzburg. As already mentioned, the Rick Steves walk (in the guide or downloaded to your phone) is very good.
A walk from the train station to the Old Town along the river offers very nice views of the Old Town and the fortress; it takes about 30 minutes to get from the train station to Mozartplatz. I personally would take Saint Julien Straße toward the river, cross the river using its bridge, and then walk the pedestrian path on the Old Town side of the river. That gives you a tiny taste of a slightly gritty area of the new town (the few blocks on Saint Julien Straße) prior to seeing gorgeous views of the Old Town. The Christmas Market is fairly large. I like it a lot.
There are nice views over the city from the fortress, as well as a small, cute (but uninspiring) Christmas market there.