As many have said, tires are really important. A good illustration of this can be seen at
In order of priority I would put tires first, traction control / stability control second, and 4WD third. The point made earlier about driving in hilly areas is a good one though. That isn't a problem I face, and I may view things differently if I lived in the mountains.
Another point worth mentioning is that tire pressures are more important in winter. In summer we (aiming for economy) rarely if ever approach the traction limit of our tires and can over-inflate the tires without serious consequences. This is not true in winter. Generally I run much lower pressures in winter and if necessary drop them very low (maybe 10 psi) to get traction where others get stuck.