E.C.
I don't have a 4 year-old; however, is he an older 4? Maybe he is teething. We just came back from the dentist with my 2 year-old. I asked shen she would get her next set of teeth and they said 5-6. Just a thought.
Also, I remember doing this as kid as well... chewing on my hair and nails. Maybe it is just something kids do to keep busy?