I have a different view from most of the other posters. If you feel uncomfortable with the idea of your little one being away from his family for many hours a day with one or two busy adults who are juggling the needs of a lot of young children, that isn't necessarily a sign that you are an overly anxious mum.
Maybe it indicates that you are in tune with your child, and that you know what he needs, and full-time school at four isn't it. We parents are told to trust our instincts about our children... right up until they reach the usual nursery or school starting age. Why is that? Do our instincts suddenly malfunction after three or four years?
There's a clash between what you're feeling is best for your child, and what's usual in our society. That doesn't make you wrong.
You do have other options. Your child can go to school part-time until he reaches "Compulsory School Age" in the term after his fifth birthday. This is an absolute right and the school cannot say no. After that age, part-time attendance can only continue with the headteacher's agreement.
Or you can delay his start altogether until you think he is ready. By law, he does have to receive an education from the time he reaches CSA, but that doesn't have to happen at school. You could home educate him for a year or two or three, and then send him to school at a time which feels right to you.
People imagine that a child who starts school later would be on the back foot, several years behind their peers. Some posters on this thread are telling you that you're already leaving it too late if you wait until next year, that you need to prepare your child by sending him into a group setting now. But there is a world of difference between a competent confident older child and a younger one. Big kids already know how to open their bottle, use the toilet independently, ask for help when they need it, make friends, and wait patiently. School is a better fit for them when they are older.
If a four year old really does need advance preparation for Reception - which itself is meant to be a gentle introduction to the school environment - then maybe Reception isn't a very good match to the needs of four year olds after all. Instead of starting even younger, what about later?