I'm in the same boat as my 3rd child sucks their thumb and gets a lot of comfort from it. I figured when he starts nursery in Sept, he'll be using his hands more and have less time for sucking.
I have older twins who were both thumb suckers. At the age of 4, we heard about Thumbsies which are fabric thumb covers. They can pick their own design, but as it's velcro, they can be removed easily so your child needs to want to stop sucking their thumb. Did it work? Yes and no! One twin stopped sucking his thumb. The other still sucks his at age 7, albeit less than he used to.
He wears a Thumbsie at night still but we catch him sucking his thumb when he's bored/tired.
For my 2.5yr old, we tried Thumbsie, but he just ripped it straight off. We also have tried other products... Dr Thumb which is a plastic cover, harder to remove, but less comfy. And another thing called a Nipit which is like a brace on their arm which stops the thumb reaching their mouth comfortably but leaves their hands free for normal playing etc. Nipit does work - my toddler did stop sucking his thumb when wearing it, but I wasn't that keen on him wearing it day and night, and felt like it was going to be a very long term commitment.
I'm going to have another go to try and break the habit before he starts school, as judging from my elder son who still sucks his thumb, it is a very difficult habit to break for some kids.
Hope one of these products can help you x