Young children use thumbsucking as a way to comfort themselves.
Forcing a child to stop can be more stressful than letting a child stop on their own. If there it becomes a concern the parent can check with the child's dentist.
Anger and frustration are common emotions in children these ages.
As their anger begins to last longer 4 year olds will deal with it by physically fighting. 6 year olds are more hurtful with words & making fun of others.