Anybody have a GOOD suggestion for getting my for 4 year old to stop sucking her thumb ?

What's weird is, as a small infant, she did not do it, and she really did not want a pacifier. She STARTED sucking her thumb at like...10-11 mos, and it is now CONSTANT. As soon as the thumb goes in, the other fingers go in her hair...we have tried "THUMM" - like a pepper liquid you paint on, ignore (the thumb sucking, not her), punish, take toys away....at my wits end. The dentist has told us it would be good to get her to stop, because he can see where her thumb is pushing into her palette, adn if she stops NOW, it is not SO bad, it may correct. HELP PLEASE.....