J.H.
I agree with the others - that is a LOT of change for a little boy to be dealing with. It sounds like your son has had different teachers because he has moved to a different room. (Also, did the other teacher leave? That is another situation that is hard for kids to deal with.) That is different than a new teacher in the same room. When students change rooms, there is an expectation that things will be different and teachers expect that it will take time for the students to get use to the routine. It is a little different when the teacher changes in the same room. Although we can process that the new teacher will be different (different ways of doing things) even though she is in the same room, a 4 year old isn't necessarily able to do that. The old teacher may have known that your son needs more transition time/warnings of change in activities and perhaps this one isn't doing things the same way or has changed some of the activities around so that she is more comfortable with it. All of this added to a move - even a happy one, is a lot to deal with.
Really talk to the teacher, ask about the schedule for the day. Try to incorporate this schedule (as much as possible) on the weekends. Encourage her to give warnings about upcoming transitions and changes in the schedule. Also, a reminder from you in the morning about what things he will be doing during the day and what you will be doing after school with him. "When I pick you up today, we are ..." gives him something to look forward to and focus on rather than the things he has no control over.
Although you don't say, will he be going to the same school? or a different one? If it is the same, does he understand that? if it is a different school, has he visited the new school yet? You can be excited about something and at the same time fearful of the other changes that go along with it. Different room, different friends in the neighborhood, moving away from people he knows, etc. That part is scary for all of us, even grownups! I know, I don't like change at all and have a hard time for about the first 6 months - even if I wanted to make the move.
Communication with the school is key right now. Perhaps they could make a chart where he gets a smiley face for each activity or transition he makes without yelling or hitting (he still needs the reminders, both about upcoming transitions and expectations). Good behavior all day might be perceived as unattainable and the first time he messed up, it was over. This way, if he messes up once or twice, he can still earn the prize. At the end of the day, if he has X amount of smileys he gets a "prize" which could be as simple as an extra book at bedtime. After a few days the amount of smileys needed goes up by 1. This added with extra snuggle time with you and/or dad, might go a long way to easing his fears.
Best wishes!