I feel the best would be to move away from England, as the system here is so inflexible and will only look at the date of birth and not the individual child.
In my view there is only so much you can 'practice' with a 4 year old. If they are not interested in reading and writing then it will just put them off. YR is meant to be play based, however there are expectations as well, e.g. phonics teaching, writing etc. Besides, how is your 4 yo dear boy going to feel if everyone can do stuff better than him. True, most will catch up eventually, in Y1 or Y2, but why put them through it.
If I had another go with my DS (now 7), I might home educate for a year or 2, or fight to get him deferred a year. DS is an April baby, so not the youngest, but in terms of maturity he may as well have been.
I wish they would change the system and decide on an individual basis whether a child is ready or whether it would benefit from staying in pre-school for another year. That would have made my DS's first school years a lot happier and it would have left him feel more confident. DS (Y3) is catching up now but is still struggling with confidence.
On the other hand, my DD (4 in October) is now at the same age my DS was when he started YR - what a difference!! Her listening skills are really good, we started reading chapter books, she loves drawing(with details), started writing her name, other words and is interested in reading. In pre-school she is one of the oldest and most able now, so that is great for her confidence. She will be fine when she starts R in September.