It sounds like your son might be dealing with Trichotillomania - this is an impulse disorder frequently seen in children (although the "typical" age of onset is 12, however I have personally worked with children as young as 4), and is similar to obsessive compulsive disorder.
A major contributing factor to Trichotillomania is stress. Many people stop the activity altogether when they are removed from a stressful environment. If your son continues the behavior after camp is over, you should have him be seen by his regular doctor, who may recommend therapy (while 4 seems young for therapy, child therapists can do wonders).
For additional information, see:
http://www.trich.org/
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