, good question. Christmas is always a bit of a gamble.
With a 4 hour drive from Zurich there is a lot of space to cover. From the Arlberg in the east, Verbier in the west, St. Moritz in the south east and Andermatt and Engelberg just around the corner. Just to mention a few. Even Cham and Courmayeur do fit in the 4 hour max. The good thing about flying to Zurich is that you will have a rental car with winter tires.
When flying to Milan you will have easier access to the Monte Rosa and the Piemonte compared to Zurich but from experience I know it is really hard to get your hands on a rental car with winter tires.
Does this help?