I think a lot of it has to do with how socially mature your daughter is. I've taught a lot of kids lessons, and because our ski area is so small, I have the ability to observe all of them every time I'm in the beginner area. Over 95% of the 4-5 year olds do just fine for either a half day or whole day. The more social your daughter is, the better she will do with a full day. It will seem like play time to her. She won't necessarily be expected to just ski the entire time, although it's amazing how many kids are having so much fun, they don't want to stop.
Generally, they will be taking at least one break in the morning, then lunch, then a break in the afternoon. The kids instructors understand that kids that age just need to be played with, and encouraged to have fun.
I would guess that 75% of the kids we teach in our childrens' program do the full day, and when you look at the sheer numbers of kids, very few get major separation anxiety.
But you know your daughter better than anyone.
As others have suggested, once you put her in ski school, you need to go away, so that she doesn't see you at all. If she starts to get some seperation anxiety, and she sees you, then it's all over, and she'll be a complete terror. Just leave. Be back 10-15 minutes before the end of the lesson. From an instructors' and kids point of view, there is nothing worse than a late parent.
Also, after her lesson is done, be sure to get input from the instructor as to how she is doing, and be sure to take a run with her, so she can show you how great she did!
I have yet to do that with my 3 year old, but I'm guessing it will be the best run of my life (even if it takes a half hour to ski 200 yards)