This stuff bothers me a lot from a philosophical standpoint. And here comes what is sure to be a long-winded commentary.
On one level, I do understand it...and this is the only level I understand it on. Many parents put their children into a Montessori school for the very reason that they hear it's "better" and they want their 6 year old to be able to add 4 digit numbers, rattle off the names of countries, and read "A Tale of 2 Cities" while discussing the impact the book has on economic, political, and social understanding of our time. OK...I haven't found the last one yet, but I'm willing to bet there is a parent SOMEWHERE in the world that expects that of their child. And until you interview every parent to ask them that question, I'm holding to that assumption.
So I understand it where many parents expect to cram a 3 year curriculum into a 1 year school year. Their child goes to your school and they spend all the time in practical life and the parents start to freak out. The child has a bad experience, the parents have a bad experience, and the school is left to explain what purpose practical life serves to a parent that only wants to see the academic results. (Even with a 6 year old, you have to build a foundation. No matter how big a wall is, it is easy to knock down if you don't build it underground too...the stuff you don't see)
On another level, I really don't understand this. Especially for a 4 year old and not a kindergartner. It is not our job to bring in children who are exactly where they should be at the start of the program. It is our job to help screen people and see if the program might not be suitable for them (A teacher might not be fully capable of helping a student with Downs Syndrome, for example. Or extreme Autism). But it should not be our job to determine that a child cannot enter the program because of age.
If NOTHING else, what Montessori is designed to do is take a child where he or she is and help them to move forward socially, academically, and spiritually (I say that without necessarily including a religious meaning to the word). Our goal is to help the child learn how to make discoveries.
In my opinion, a school that says no to a 4 year old is doing it for one reason- the convenience of the staff. This should be the last person convenienced when there is a challenge. A 4 year old starting is a bigger challenge than a 3 year old starting. There's no question in my mind about that. A 5 year old is even more of a challenge if he starts late. But isn't that our job as Montessori teachers? To take the child and help the child to normalize?
I understand that you won't get the full benefit of a Montessori classroom in 1 or 2 years. Is it our job to make sure students get the full benefits? If a child is sent late, the parents recognize that they might not get every single benefit, but they want what benefits they can get. They want their child to think on his or her own. They want their child to have a love of learning. They want their child to explore. Any Montessori teacher should want that for ANY child.
Now, it may be that a school is so well run that they have few, if any, people leaving and they stay the full 3 years. As a result, they don't really have many open slots for a 4 year old because no 4 year olds left and the new 3's are coming in. That's a different story than a simple blanket policy on not accepting a certain age group.
But to me, in the early years at least (elementary -- I have a slightly different opinion on), to say "we cannot accept ____ age group" is completely contrary to the Montessori method.
Just my 2 cents. Of course, I wouldn't type all this in your letter. You don't want to say, "I don't think your policy is very Montessori." That will get you nowhere. :-P