My 4th year old baby always his thumb in his mouth . i try but he not leaving his habit. please give the solution. ?

Distraction: Try not to focus on the behavior but when you notice it occurring try to distract him with activities. Placing a non toxic product such as Tabasco sauce on his thumb may discourage thumb sucking. He is old enough for a reward system as well. All the best with this. Some adults still thumb suck. Although I dislike pacifiers the benefit is, you can take it away.