Sorry you are going through this. Hopefully you will get some good feedback here. I have a DS4 as well and we have had difficulty with preschool as well and put him in a smaller pre-k with more one on one. We put him in this setting because he was acting out in his other pre-k and they didn't know how to handle/deal with him.
In his last setting he was extremely overwhelmed as there were about 20 kids in their other program and they had no behavior management system, and no clear limits. We had thought it was a good program because it wasn't academic and more social/play based. But we quickly learned that wasn't enough for him. They actually asked him to leave and said (in a voicemail on my phone) that he couldn't go back because he wasn't behaving. They also suggested that we refer thim to get special ed services because they weren't sure how he could ever make it without a 1:1 aide. Anyhow, I could go on and on about how they screwed things up but I won't. So we had no idea what to do and almost considered going through special ed, however I just didn't feel like it was the right move and I felt they just weren't used to a child that was as gifted as he is (and I had already had him evaluated at 3yo by a developmental pediatrician because his pre-k teacher then said she felt he needed it and that he could have "something on the spectrum....but not aspergers or autism" LOL). Anyhow I figured it wouldn't hurt to get him evaluated when he was 3 because at the time I was a mental health therapist with children so I knew he didn't fit the criteria of anything...and I wanted confirmation that there wasn't anything else going on and the reason for his behaviors was that he was gifted. And the results were "he's very bright...he doesn't at all have aspergers." We also knew it wasn't an issue because he didn't have any of those behaviors in other settings or at home.
So we found a smaller setting just 3 days a week and it is is so much better. He has been there about a month and a half. We can't afford Montessori right now, and plus they have no openings. His behavior has been good here because they have challenged him more and have limits in place. they feel his behavior is the same as his peers, however they notice how different he is. They do say that they can work with him individually at his level but they do worry about how he will do in school because of how advanced he is and how many more kids there will be and less one on one. With my DS he tends to act out when bored and was not as much internal. However, one thing that may be similar is how he interacts with peers. My DS doesn't talk that much with peers his age, because he is so different then them...they often don't even respond to his comments or questions. However, he does communicate much more verbally with adults or older children (like children that are 6+). I am just curious if that may be a factor with your son. Also, my son doesn't really seem to know how to interact much with peers his age and tries to make them laugh like yours does. It is pretty easy to do, but he tends to act with them like he does with his little sister (who is 1 1/2). He does silly stuff. He has been doing better with this though. I have noticed though that he doesn't at all do this with older children. A psychologist I consulted with who specializes in working with gifted children said that he tends to interpret their intellectual age and responds that way with his play with them. I find this very accurate. Unfortunately sometimes he assumes that they are on a lower level then they actually are. Anyhow, I have gone on way too long. Just wanted to share my experience and let you know I can relate to how you feel. I would get all the knowledge and support you can. I don't think evaluations can hurt at this age, but I agree that you want someone who is experienced doing an evaluation and that they get information about home and school as well.
Good luck in your upcoming decisions. And feel free to send me a message sometime. : )