4 days = 3 nights, you don't have much time. That would be a maximum of 2 places. For q whirlwind vist i suggest:
1 night Luzern. Go there direct from Zürich airport (~70 minutes by train).
2 nights Jungfrau Region of Berner Oberland. Places to stay: Mürren, Lauterbrunnen, Wengen or Grindelwald. From Luzern you get here by train via the scenic Brünig pass line.
From there, train to your exit airport (Zürich or Geneva).
Zermatt is too far south for this short trip.
The Glacier Express runs east-west through the Alps between Zermatt and Chur or St Moritz. Unless you incorporate it as part of a tour, it is not convenient.