From what I know about chicken pox, they usually start on the torso and spread out from there. If he has none on the torso, I would guess it is not chicken pox. My husband had it a few years ago (awful experience for an adult especially) and they all started on his torso, spread to his face and head, then to his limbs. It was about 15 hours from the appearance of the first blemish until we figured out what it was. We then kept him away from our 4 1/2 month old son, but it was too late. 10 days after it started on my husband, our baby broke out. Started on his torso also, then spread to his head. It was very scary to me as a new parent, but he only got about 25 blemishes total, only one really blistered up very big (on the top of his head) and he never had a fever or complained one bit. I have heard that it is typically very mild for babies. I would be worried about a 4 week old getting it, that is just sooo young, but it sounds like what your son has is not the chicken pox.