Nothing is a substitute for four winter tires on your vehicle.  Not four wheel drive, not two winter tires, and not having awesome winter driving skills.  While these and other factors are important, the coefficient of friction is chiefly what determines your ability to decelerate, accelerate, and turn your vehicle.  When temperatures fall below 7 Celsius (45F), winter tires begin to grip the road better than all seasons because winter tires are softer at lower temperatures and can still mold to the grooves of the road when all seasons have become rigid.

Here are some examples of how important winter tires are: