sb34 - How did you feel about his teacher before all this blew up? Do you think you have a good relationship? I hope so, and please, please, try and maintain this. I think a lot of what Ghosty says is very helpful - when you go in to school it could VERY easily turn the head and the teacher on the defensive - they are only human after all. When you go in make it clear that you are on the same side, that you admit that his behaviour is unacceptable, that you are pleased that the school is communicating with you, that you are interested in what is going on, that you are not trying to make excuses for his behaviour, but that you would like to work together for the future to address the behaviour, and any underlying problems.
I know a report book at 4 sounds AWFUL, but it needn't be. It could be a very effective way of making sure that you and his teacher know exactly what is going on - and that your son knows that everybody knows ... It can also be easily turned into a positive/reward thing, if praise is given as well.
We were fortunate (!) to be phoned by the Head, and summoned into school several times after ds1 had only been there a couple of weeks - it was DREADFUL at the time - I cried for hours, and worried during the nights, BUT we used the advice I've just given you, and now have a fantastic relationship with ds1's teacher, and the Head. DS1 still has a few hiccups at school, but only a few, but I know I still get to hear about everything, because his teacher knows that I want to know the full situation. Parent/teacher meetings are week after next, but I know there won't be any surprises.
Anyway - hope that helps a bit. Hope you manage to relax and enjoy the weekend - fab weather, and hope meeting is very positive on Monday.