Here is a good article for you, about this subject:
Next, if your ideas about how the kids are handled/taught, does not coincide with the schools/Teacher's philosophy... then perhaps you need to re-evaluate that.
And if your son is playing with older kids (obviously it is a mixed-age) type of school/classroom... then it does not seem to be real positive for him. Not all schools are "mixed-age" classrooms.
Next, your son's Teacher(s) seems to do nothing about it nor how the kids play. But, there are some teachers who WILL use it as a "learning" occasion, and talk to the kids about it and how to behave and be "nice" etc.
So that is why I say, if this school does not coincide with your ideas of how kids are taught/treated... then maybe find a school that is more constructive.
ALSO at his age of 4.5 years old, I believe that an actual PREschool (not play based play school), would be better for him. It is more structured then.
Then again, throughout his school years, and as he progresses onto other grade levels... there will unavoidably be other kids/varying ages/different influences there, that will impact your own child.
So, you need to teach him, about values/right&wrong/how to discern situations & good or bad kids etc. And how to problem-solve if a kid is mistreating him. For example, telling the teacher. A good Teacher, will assess the situation without assuming blame without analyzing the situation, for example.
Throughout school life, these things will happen. And it may be a cliche, but it starts at home... in how a kid, will be able to handle themselves, make decisions for themselves, choosing the right choices etc... and that their Parent, is there to advocate for them in instances of bullying or blatant misconduct upon them by another kid or Teacher. That the child have a voice... and will know they can tell you, the Mom about it or any mistreatment.
anyway, sorry for rambling,
all the best,