Hmm, well the folders I use for whittling are pocket knives (a little shorter than 4" closed). I've been using swiss army knives for a long time but sort of recently I got a Case medium stockman which I find a lot better for whittling. If you're looking for that kind of folder than something like a stockman pattern would be a good choice because it has three blades to choose from, normally a clip, sheepsfoot and spey but I got one with a pen blade instead of the spey because I think it's more useful for whittling. Only problem I see with that pattern is the sheepsfoot blade sticks up quite a bit in the handle when closed so it can be uncomfortable. There is another pattern called a Texas Jack which has the same shape as a stockman but only has the clip and pen blades so it's probably a little more comfortable to hold.
On the other hand, if you're looking for a larger folder then maybe something like an EKA folder would be good as it has a scandi grind, I've never used one of those myself though. I also find a larger knife a little harder to use.
Like Robin said though, a fixed blade carving knife is probably the best, I think it's because it's stronger, won't accidently close on your fingers and has a larger more comfortable handle. I just got a Mora 120 for christmas and boy is it sharp, it carves effortlessly. But I'm still going to use my pocket knives because they fold up nice and small so I can bring them with me more easily and whittle while out and about. You could still do that with a fixed bladed knife but it's a bit bulkier, plus it's a fixed blade and illegal to carry in some places.
Oh and by the way, I'm not that great of a whittler so keep that in mind, but I think I'm alright.
Case medium stockman I have this one.
Texas Jack
EKA folders
Moras The carving knives are near the bottom.
