It can be used on just 1mm of ice, if you can see you are likely to loose traction or you feel it loosing traction ( feels similar to a slipping clutch ) you will be fine in 4wd as the slipping action will dissipate the wind up in the axle and transfer box.
Deep snow your even more likely to need 4wd.
This road was enough to cause us to use 4wd a few weeks ago, its a layer of ice under the slush/snow
