Your four year old sounds like my five year old (who's also in Montessori!). He also likes to play with kids older than himself sometimes, and they are rough on him.
My suggestion is for the most part to let it be. So long as the kids aren't doing serious harm to him, this is a great learning experience for him; and he may simply enjoy the physical, rough and tumble style of play. If he's enjoying it, even though sometimes he might cry for a bit, it's not doing him any harm, and he can learn how to navigate social situations with people bigger and older than him.
Talk to him about it, though, and find out why he likes playing with them. If he's coming back from a play session sad, talk to him about why he's sad. Ask him whether he actually does want to play with these kids, or if he wants to take a break.
Also talk with him about why he chooses to play with the older children. He may not yet fully understand, but he may; my son did at that age, at least partway. He may like the attention of the older children, but my feeling is he likes their maturity. If he's significantly ahead of grade level, he might like being around children who are intellectually at his level; so it's possible he sees this as his only option to interact with his intellectual equals. If that's the case, you might be able to help him find other friends to play with.
Most of all, give him some tools for handling things when they get too rough.
You're playing too rough, please stop.
You're playing too rough, I'm going to go take a break.
I don't like being hit in the head, please don't do that.
It hurts when you kick me, please don't.
These sorts of tools are very important to learn, and they're things that he'll need later in life.
Also make sure he knows that it's okay to take a break when things are getting too rough, and make sure he knows that it's okay to ask for an adult's help if things are getting too rough. It's important at this age that he learn to do these things - you're not going to be around when he's playing on the schoolyard, and as he gets older in other places where there aren't any adults acting in loco parentis. He needs to learn how to handle these situations when there is a safety net, and you or a teacher can intervene if things get actually dangerous.
Finally, as you say he's in a Montessori school, some specific things for that. Montessori education is whole-person education, and they focus very specifically on the social development. I don't know if your child is in a school that fully embraces the Montessori method, but if he is, the teacher should be teaching how to handle these situations as part of the education. My son's teacher focuses on expressing emotions in a calm and direct way, and encourages the children she guides to handle conflict in positive ways. Work with the teacher to find out how she or he guides the children through these kinds of scenarios.