You should use 4wd in the snow as fast as you would be comfortable driving in 2wd. It's usually recommended to stay at or below 55, but that's also just common sense, IMO. If it's slippery enough to "need" 4wd, it's slippery enough to slow down to a reasonably safe speed. Think "How much longer will it take to stop on this surface" and try to adjust so you'll still be able to stop in a decent distance if necessary.