DS is August born and definitely wasn't ready for school less than a month after his fourth birthday - he's very bright, but was in no way interested in sitting still and drawing, let alone trying to learn to write.
Luckily, at the time we were living in Japan, so didn't have to follow the UK system (though we could have sent him to the British School at 4, as many of my friends did with their DCs). Instead, he stayed at Japanese kindergarten for another year - lots of playing outside, singing songs, making models out of cardboard boxes, no formal learning at all.
By his fifth birthday he was showing signs of being bored with that, even though in the Japanese system he would have had another year and a half of kindergarten to go (they start school in the April after their sixth birthday there), so he started at an international school which was roughly based on the US system - school (kindergarten year, which is equivalent to reception) starts after you turn five. By then he was absolutely ready to start learning to read, and forged ahead with enthusiasm.
We moved back to the UK when he was 8, and he went straight into year 4, where he was one of the youngest and had had one year less at school than all his classmates, but that did not cause any problems at all. He is now year 9, doing extremely well and enjoying school.
I do wonder sometimes if he would be as keen on school now if he had been forced to go at 4 when he couldn't even hold a pencil properly, and would probably have struggled with the stuff he was asked to do.