"I have an almost four y-o that sucks her thumb when she wants to fall asleep. She's never wanted the dummy and from day one has been a thumb-sucker. It's not something that worries me a lot at the moment, since there isn't any dental problem (as said by the paed) and she only does it at night or when very distressed. But I'd like to know how can I get her to abandon this behaviour in the long run. Most children are weaned from dummies by age three, but it's way more difficult with thumbs ! Related or not, I was thumb-sucking until 10 yo !"
Ah, Marta and her daughter are my people! You guys know I sucked my thumb to fall asleep until I was 11, so I'm absolutely not in the "they must be off the thumb/pacifier/lovey by age X" camp.
My mom's theory (and I think my mom was amzingly empathic, especially when we were little little) was that if our needs were being met for comfort and love and snuggling, and we still wanted comfort objects, then it was just some developmental thing and we'd grow out of it when we were ready. So she defended me when my grandparents came up with all sorts of cockamamie ideas to get me to stop sucking my thumb (painting bitter stuff on it, trying to traumatize me by giving me scary mental achors connected to thumb-sucking, etc.) and figured I'd grow out of it.
I think Marta's daughter will grow out of it, too, when she's ready. It seems like it's an effective way to self-soothe, and I worry that if she's forced to stop she'll either be emotionally at loose ends or will need to develop something else to self-soothe. If Marta really wants her daughter to stop, probably the best way to do it that won't be really traumatic would be to figure out what else her daughter could do to self-soothe (holding a soft lovey, touching her own hair, etc.) and getting her to focus on that instead.
I doubt her daughter will do it in public when she goes to school and realizes the other kids don't do it. (I hear a lot of stories about kids who quit cold-turkey when they started school, except for a little bit at night by themselves.)
So my advice is basically not to worry about it and let her find her own way with it. Once she doesn't need to self-soothe anymore she'll give it up.