K.I.
I just asked my son's (also 4) Dr. this very question, just today! He just showed me his nails...which he obviously bites...and said "It's a better stress habit than smoking"
I am going the route of trying to NOT make a big issue out of it and hoping he will grow out of it...or in the very least I won't stress him out more by nagging him about it.
My son also bites his toes...and the skin...I do point it out to him occasionally when I think he has bitten one down too low!?
Sorry I do not have any answers for you...but your not alone! I worry about this very much too!