No I wouldn't use a ski nanny service. When DS was little we used a mix of daycare, ski school (when he was old enough) and skiing with us. Those that I know who have used ski nanny's have taken their regular nanny or a staff member (on annual leave from their regular daycare) when their children were small and they did this because their children were familiar with the nanny that they took. Some children, especially (preschoolers) settle really easily with unfamiliar carers and others don't, ours was of the latter variety, which was another reason I wouldn't consider a ski nanny. Now that he is older we mix up ski school and skiing with us when overseas, and at home we sometimes add in a friend if there happens to be one around.
Daycare provided us with pagers, which they used to contact parents if children did not settle (we got through the long queue and to the top of the excruciatingly slow first chairlift before it went off on one memorable occasion). I think you would consider how you dealt with this, do you require parents to carry a cell phone if they use your service, or will you have pagers? Also what will you do about food? four hours is too long for most kids to go without a break. Or what if the child just didn't want to ski or it was too cold, where would you go and/or what would you do? the benefit of the daycare we used when DS was little was that he got a private lesson (from age three) that lasted up to an hour, but if he wasn't keen then it wasn't pushed.
And I agree with geargrrl, it often isn't the kids that decide the terrain to ski, it is the parents, and sadly it often isn't dictated by what said parent would like to ski (otherwise those same kids would be on double blacks), often I think it is dictated by the parental "comparatition" "my kid is six weeks old and hucks 20 foot cliffs, mentality, so you would also have to think about how you would handle parental expectation about terrain.