4 yr. old anger

My 4 yr. old son recently has been violent at daycare and even my grandparents.  At school he is screaming out at other kids & teachers, has tried to throw chairs, hit his teachers .  He recently has even started threatened to hurt them or kill them.  He does not talk like this at home, I'm not just saying that either.  Usually it starts over very simple things and he just looses it.  5-10 minutes later he acts like a normal 4 yr. old and like nothing happened.  I do notice that he doesn't like being in large groups if we go somewhere new he will not speak to anyone he does not know and be very shy but almost a mean shy.  For instance last week we ate dinner at his grandparents and as soon as new people arrived that he didn't know he went in the other room & refused to come out.  It's almost like being around new people or a grou[p of people gives him anxiety.  Does anyone have any suggestions on what to do or where to go.  He is normally such a good kid & all of a sudden he's just become this boy I don't know anymore and I can't keep leaving work to get him.  I just need some advice on what is going on.  Also he has been at the same daycare since he was 6 mo. and just 1 month ago they moved into a new building and thats when it seemed to get really bad.  HELP!!!