4 year old boy likes to take off clothes. I guess it's called exhibitionism. Anyone else experience this?

Not necessarily. All kids have periods when they do things that are not what we would like them to do. You do not mention where he does this. Home, school. Is this new? If you have already set limits, and consequences for the behavior and explained to him where he is allowed to and where he is not. And you are still having problems, have your boy evaluated.