I know nothing about Montessori but this wouldn't bother me.
I think there is a difference between taking recess away as punishment (like, if he had hit another kid and had to stay in because of it) and this situation, which was basically demonstrating natural consequences (if you don't finish your work, you may have to do it when everyone else gets to have fun).
It wouldn't occur to me to be upset about this or to blame the teacher for the child's choice not to work. It sounds like he understood that he was expected to get three things done before recess and that he chose not to. Working through recess seems perfectly reasonable to me.
As for the notion that outside play should never be taken away, kids need to run, fresh air is important, etc. I'm having trouble getting on board. He missed a day or two of recess, he wasn't locked in a dungeon without sunlight.
This is where I stand. He knew the rules, and didn't follow them. If this is an on-going issue and he has trouble getting the three things done regularly, maybe Montessori isn't for him. But, I assume parents put their kids in the Montessori program to teach time management, work skills, etc... so she SHOULD be following through with that.
Thanks for the replies. I haven't responded to her yet, but he did tell me that he had recess today. He was very proud that he had completed his work. I think I do need to keep an eye on things, though. I think I need to trust her a little more. I don't think this behavior is normal for him. He has had a really off week and I think that has had a lot to do with his little brother. His brother is teething and hasn't napped all week, so we haven't had any real 1:1 time with him; I think he is acting out because of that.
I do not like (at all) the idea of a teacher taking away a recess from a 4-year old, but I am going to set my judgment aside for the moment and will address it if it comes up again.
To answer a few questions, it does make sense that he didn't have recess on Wednesday, as it is a very short day. I had forgotten they don't do recess for half-day kids. Also, while the teachers there are certified, I don't think this a very strict Montessori program compared to some others, so I don't think it is usual to take away a recess. Regarding the work being done inside or outside, I don't know what that means. All of the stations that I have seen are inside, but it is possible they had some sensory work that could be done outside.