These are very scary if you are turning toward the right because you begin to move out of your lane into the lane of oncoming traffic. In this case ease off the accelerator and steer gently toward your lane. At first you won't get any response from the steering wheel, but as the car slows the steering will respond. If you save it, take a deep breath and slow down. Four wheel skids are also caused by heavy braking.
Hills are often a source of frustration to drivers when the roads are snow covered.
Good tires are a must, though much can be gained with driving techniques. As you approach a hill pull the gear shift lever into 2nd range. Accelerate a little as you do so. You don't want the car to shift half way up the hill or you will lose it. Now accelerate a little more, just enough to feel the tires begin to spin.