It sounds to me like you've already made up your mind, so I don't know that anything I'm going to say is going to make any difference. However you did ask for advice so...
1. I don't get the "boy thing". I have heard it before and seen it in action. I think kids should be able to sit still and follow some simple instructions. I realize that some kids (boys and girls) are higher energy kids and fidget. However if the preschool teachers are suggesting that your son's behavior, I think you should sit back and let them decide what THEY think after the therapist does her thing.
2. I don't understand why you're upset that they had someone evaluate your son? I respect that you asked to be informed, and would be a little annoyed too. However from your post it sounds like you're upset she observed him and made her opinions without your input. I think that's how "observations" are done. They don't want the kids coached, coaxed or otherwise acting different. So if they informed you in advance, maybe they thought it would taint the the observation?
3. It sounds like they've come to some conclusions you clearly disagree with. So why not talk with your pediatrician about their "findings" and see if you can get referred to a third party that can perform their own assessment of your son?
4. On the subject of the kicking and biting. I think it happens. (Doesn't your son fall and get hurt occasionally on your watch?) I think they did their job informing you it happened and don't expect them to elaborate on what's happening with the other child. I think you can ask the teachers/director of how these situations are normally handled, but don't expect them to discuss this child who bit your son as I think they view that as private. Just as I wouldn't expect them to talk about your son's perceived sensory issues with any other parents.
Overall, I think it's perfectly natural to feel defensive of your child. Especially if you feel that these teachers are off base. That said, I think that you should take their assessment seriously and follow up on your own. Sensory issues are not something to be taken lightly. If your son does need therapy it's best to get it for him now...the sooner the better.
If you really don't like the teachers or program your son is in, by all means start summer vacation early. Just realize they may expect you to pay for part of the program since they aren't going to be able to fill your son's spot for the rest of the year. This is pretty normal.
If you're starting him in another preschool next year, then I would be up front with the director of that program before your son starts. Maybe they can give you some tips or advice on how to help your son be successful in school...practice sitting still, following instructions the first time, writing his letters and numbers, etc. Of course a four year old isn't expected to do this all WELL right now, but practicing with him now will help make sure that going back to school in the fall isn't such a big culture shock to having Moms undivided attention.
Best wishes to you mama. I know it's difficult watching your child adjust and have things happen when you're not around. I hope your happier with the next preschool.