My DS was similar in that he was totally ready, academically, to start school. He was also quite mature in an 'able to sit still and listen, wanting to do things right' way. Emotionally he was mature too, confident, good at managing his feelings. Nevertheless, he initially found separating from me at drop off a bit hard - but nothing that couldn't be managed by going gently.
However, he was 'socially' immature, inasmuch as he didn't know how to 'be' in large groups of children without close adult supervision. Which needn't have been a problem by itself - this was what we were hoping for him to learn in his reception year. However, circumstances then were that there was a bit of a bully in his class who was nearly a year older and 'experienced' in group status 'games' and DS was an easy target.
Now being bullied is terrible whenever it happens but at just four, and as a first experience of social peer interactions (outside of situations with a lot more adult guidance e.g. nursery school), I believe it is indeed strongly 'emotionally harmful'. Basically he learned that that is how children behave towards each other. It was very painful to watch and took a lot of time and hard work to get over this (some effects I believe are still there, nearly three years on, e.g. some social anxiety).
In hindsight I wish we had deferred his entry to e.g. February. Another child did this, and settled beautifully into the class where all the nasty hierarchy struggles had been completed already.
We had considered deferring, but like you the circumstances at home were such that it would have 'cost' us a lot. We really needed him to be at school full time. If we had known the cost to him, we would have made it possible somehow.
However, we were unlucky in that we encountered this bully who targeted DS. It could have turned out fine.
So a bit different than your situation in that our worries about DS were about something different. What I would say, in your situation, if you do decide to send him to school from September, then talk to the school beforehand, really emphasise his issues, and discuss ways to proceed. It is likely that school will think won't take your concerns very seriously initially - probably every other parent is worried about things like this, most children end up being fine. But if you really feel your DS would profit from e.g. a gentler start, then don't let them convince you otherwise! If everything goes well you can always move to 'standard'. The other way, yes he would probably be ok after a while, but maybe only after emotional damage has occurred. And are you happy for him to be just 'ok'? When he could be doing really well and being happy?
At the end of the day, listen to the school/teachers but if you are convinced that a gentler/delayed start would be better for your child, then fight for it.
A friend's daughter started part-time because the parents were convinced that she couldn't cope with whole days. By the first half-term she was asking to stay for the afternoons. Ok so maybe she would have been fine going full time from the start! However, there was no harm done by going gently. The other way round might have been ok as well, but equally might not!