4 years old is too old for a paci which I'm sure you know.
Best approach at this age is to take it away cold turkey even if he cries and screams for it.
He'll eventually forget about it.
If you're not up for that approach then you can try putting something sour on it like lemon juice. He may not like the taste and give it up.
You could also try and cut the nipple off little by little just a bit here and there until there's nothing left.