I too have struggled with this dilemma recrntly, my son is four in June and has a place for this September, However we have declined it until next year. I work in a childcare facility and having spoken to a lot of parents about this they all agree its best to hold them off. AS someone mentioned before boys are slower to develop emotionally and socially.
Also think of this, the legal ratio for a MOntessori class is one adult to eight children, and essentially this is still a "creche "facililty in that the staff are trained in childcare. I'm sure teachers are too to a certain extent but you would want to be absolutely sure that the child could cope going from a very small class of Montessori children with at least two teachers in teh room (legal requirement) to a much larger class with only one teacher. From my experience as a childcare worker and manager it is best to hold them off for an extra year. I knew one child who went to school at four, only for his parents to decide in December it wasnt right for him .they pulled him out and sent him back to Montessori. Did quite a biy of damage, this set the child back at least two years, he became clingy, disruptive etc.
I think you do more damage sending them too soon than holding them back......At least you will be sure the child is completely ready.
HTH!