Thats' because a 4x4 will get you moving and keep you moving but it can still slide and lose control if driven incorrectly (too fast, too hard accelerating, too hard braking, too quick on the steering etc). It will also still lose it on ice. What's worse is that the 4x4 is then often much heavier and therefore much less likely to stop once lost control.
4x4's are great if driven correctly but too often they are driven by morons who think that the 4x4 makes them invincible. :angry:
I hate to say but it is generally the moron in the brand new 4x4 who has more money than sense that will fall foul of this way of thinking (sorry to all you brand new 4x4'ers out there but talking in averages here

)
....light the blue touch paper and run.......