You can see thumb sucking as a self-soothing mechanism for coping with stressful situations. At age 4yr they may go to day care and Kindergarten/Prep is around the corner and there is a shift in our expectations. Other things what may play could be a move, a new sibling or other change in circumstances.
Initially I wouldn’t be too much on top of it, other then a calm, loving and patient approach and make sure your child gets 1on1 attention and gets confirmed on how important he/she is

My daughter started before 1 as its a comfort/soothing thing for her even though she has a blanket. I was told to just leave them to grow out of it

It’s definitely something he will grow out of. My son used to be a thumb sucker, he broke the habit when he cut his thumb and it was too sore to suck. You could try that, but I wouldn’t recommend it lol ????

There are a number of ideas on pinterest including phrase reminders, reward charts etc

Usually you have to replace one habit with another, but I’m not sure what to suggest you replace the thumb sucking with. Maybe give him a toy like a yoyo that he has to keep fiddling with and that might distract him from thumb sucking.