DS is August born, is the youngest in his older-than-average reception class. You know how they say 'your child won't be going to school with 29 kids who are nearly a year older than him' - well it's not all 29 other kids obviously, but in his particular class there were 10 who turned five before the second half of the first term started... and now there are 20 who are 5+ whereas DS is still only 'nearly fourandahalf' (as he informs us frequently).
Academically, he is doing really well. No worries whatsoever. If anything, he isn't being challenged enough, because there is hardly any differentiation - they are being taught to the 'average' and he is beyond that. I don't mind, as I feel that at that young age, school shouldn't be about being challenged all the time. He is having fun and playing and learning/making progress.
Socially and emotionally, is a more complex story. He began wetting himself because he didn't understand the implicit rules (e.g. 'yes actually it is preferable for you to disrupt carpet time by asking to go to the toilet, than to disrupt carpet time by peeing on the carpet'). He was extraordinarily tired. Lots of what happens socially in his class, e.g. friendship hierarchy competitiveness, goes totally over his head, and left him quite friendless initially (until he found a fellow loner to team up with). He was sad and felt left out and didn't understand why some of the other children were unkind to him. Being very keen to join in, he began copying the older children's behaviour, without understanding. E.g. name calling; thinking it was just something funny; then being totally bewildered when being told off. Some of the other children are into things e.g. Star Wars that DS IMO is to young for, but DS desperately wants to join in and has a double struggle as he a) doesn't know what they are talking about and b) even when he does 'know', he doesn't 'understand'.
At the same time he found settling in easier than many of the older children, some of whom still cry and cling to their parents at drop-off time, whereas he skips in with a smile on his face.
Perhaps he didn't get as much support from the teachers/TAs than he might have, because he is so confident and academically on top of things; so it is easy to 'forget' that he was only just 4.
My 'advice' would be to focus on social confidence - things like asking for help, indicating preferences, and letting people know when he is unhappy or confused.