the law says nothing about being a 4x4...as well as having to carry chains if you are not a 4x4,
the signs say "carry chains or use good winter tread beyond this point"
That said, I've always used all-seasons on a 2 wheel drive vehicle in the past with no issues, and I've been on the calgary-vancouver highway in some pretty nasty weather!
The most important part is knowing how the vehicle will behave, and not pushing it past it's limits... also realize that 4wd may give you more "go" but it actually gives you LESS "stop"! (all vehicles have had 4 wheel brakes since the 30s... and the heavier 4x4s take longer to stop!)