@Bee haha when they come in
Honestly his thumb won't hurt his teeth when they come any more than a dummy and at this age you're not going to stop him at this age. You could try mittens or something to make it harder and replace with teething toys he can chew or suck instead. My son always had his fingers/thumbs in his mouth when teething so unless it's something he's always done it could be a sign of them starting
I tried mittens because mine is a scratcher and she used to get so, so mad. There really isn't a lot you can do at that age for it. Don't worry about the teeth. Yeah, advice says it can be bad for them but we're talking years in the future, and only if it becomes a seriously habitual behaviour. A 4 month old is not going to ruin teeth he doesn't have, and you're not going to stop them sucking on things any more than you'll stop a cat from scratching. It's self-soothing and figuring out their bodies and environment and it's completely normal developmental behaviour.
My baby is 6 months. Been sucking her thumb since 3.5 months. It's her self soothe strategy and I'd personally prefer it to a dummy. She sucks her tongue too. She'll grow out of it.
What teeth? 😂