I really think you shouldn't worry about it and let your daughter suck her thumb if it makes her happy. I have a 5 1/2 year old who is just now deciding on her own that she is going to stop sucking her thumb. At around age 3 or so she limited her thumb sucking to in bed or when she's really tired or watching a movie. I had told her you aren't allowed to suck your thumb at school and she never did. I'd always asked her teachers at pre-school if she sucked her thumb at school and they said never. I also have a 3 year old daughter who loves the pacifier, but, again, it is limited to the car or sleep time. And, I have a 6 month old little boy who is starting to suck his thumb and I'm so happy. Since I have both, I can tell you the thumb is better! Let her do it and know that it makes her happy and someday, she will stop.
And, by the way, our pediatric dentist is not worried about it at all. He tells me to not worry about it and she will stop eventually when she is ready and I really think it is true.