How can i stop my 4-year-old from thumbsucking?

Stop thumb suck: Limit the time s/he sucks their thumb to the bedroom inside the house and not outside. Praise when s/he is not doing it. Help them understand that when they stop, you will be there for them. You may allow thumb or finger sucking after being hurt/injured. Make them aware of the behavior. Supply an alternative object of desire. They will eventually outgrow the behaviors. Kids eventually give up.

Options: There are many things you can try including nail paints and things that taste bad but they only work sometimes. They need to know that they need to stop and why, they need cofidence and a desire to stop. It is not easy sometimes but it needs to stop before 5. After it can cause dental/skeletal problems. You may even elect to have a habit breaker placed by your dentist to make them stop, it works.

Let them suck: A child this young sucks their thumb for comfort, and is part of their psychological make up. Let them do it. Peer pressure will reduce it later to non-social times and then it will just go away. If not, and causing problems it can be treated at 7 or 8.