All this "summer born children" stuff is a red herring IMHO. There may be so-called evidence to back up what they're claiming to be true but at the end of the day, you always have to look at the child as an individual.
I can only tell you about my own experiences with my late August born DS. He entred school at 4 years and 2 weeks old and was full time within 3 weeks. He never so much as looked over his shoulder once to to see where I was, whilst some of his Autumn born classmates were clinging to their mothers having to be, quite literally, peeled off them by classroom assistants.
He could read before he started school, I'd taught him, as soon as I thought he was able to take an interest, by going through phonic flashcards with him and this meant he didn't spend time doing any of the preliminaries that the others did in reception class and was able to move stright on to the reading scheme. So in that sense he was never held back by being a Summer born child.
He achieved well through primary school and finished the school gaining all 5s in his SATS. So again, no reason to feel that he was ever remotely held back by being a Summer born child.
He's now in Yr 8 at secondary school and still doing really well achieving target levels that predict he's on course for A*s and As at GCSE.
He's always been outgoing and confident too.
So all in all, I've no reason to think that he's been held back or has underachieved in his education as a result of going to school at just 4. On the contrary, he was more than ready to go to school when he did and was quite clearly far more ready than some children who were more or less a full year older than him.