My little twin grandsons were 4 on the 30th August and started school the following week. They've just completed a full school year and are still 4 for another 3 weeks. They seemed too young, but fortunately they have loved school, and have lots of friends. It does seem strange, though, for them to be assessed for their progress, when they wouldn't even be at school yet if they hadn't been born a month premature!
I would prefer that children were at least 5 before they started school. They were very tired in the first term, and would fall asleep in the car going home. They're fine now, but you can always tell when they've had a challenging day, and for 2 days a week they spend an extra hour in after-school club. A lot for little 4 year olds.