A.H.
So far my kids have not been thumb suckers; they wouldn't even take a paci; spit it out every time. (For which I'm actually grateful, one less thing to wean them from). So, I can't offer any advice on how to get her to stop.
However, I wanted to express the importance of breaking this habit. I was a serious thumb-sucker til I was 6 or 7, and my teeth suffered greatly. My top front teeth grew out around my thumb, and I have what they call a severe over jet. I was teased so much as a kid b/c of my teeth. Even as an adult, I still have kids (in all their innocence) ask me why my teeth stick out. I hate having my picture taken b/c I feel like that's all you can see: teeth! and they're just awful pictures. It's painful. It was painful then, it's painful now. I finally have braces to correct it, and my mouth hurts every single day. But the physical pain pales in comparison.
I wish I could remember what made me stop, but I think I just eventually outgrew it. At any rate, I would encourage you (and all moms) to try and break the habit. My case might be extreme, but if your daughter doesn't have to have braces or suffer the wrath of mean kids later on, so much the better.
Good luck!