I don't know the answer to that one but I would think given his age it is something he has picked up from other kids. Mackenzie started hitting and biting at 18 months after she had been 'attacked' 4 weeks in a row at playgroup by a child who pounced for no reason. She still hits my DH and i occassionally and over the last week has come back to biting if she dosn't get her own way. We have always been very consistant with the dicipline for this behaviour - explaining that hitting is not behaviour we like and the naughty corner works well for us. In saying all this the hitting other kids really only stopped about 6 months ago. It's a hard one and I think one you might just have to ride out. I know the biting really got to me and I was relieved when this stopped. As I mentioned the biting has just started on again (on us only thank goodness!) but it's only an attempt, not a 'real' bite so to speak. I've spoken to other mums about the biting and the ususal response is they grow out of it and how many 4 year olds do you see biting. I know it's frustrating for you but apart from having a response plan I don't know what else you can do.