My 4 month old son always suck his hands fingers ,even after feeding him. I m afraid of that later on he will suck his thumb? I dont know how to deal with it

Every time you see him put his hands into his mouth, take them out gently and tell him no. He won't understand and stop it immediately but one day he will know that it is not encouraged behaviour and he will stop. I did that with my girl. I don't allow her to bite her toys too. So she grew up not anyhow picking up and putting stuff into her mouth