Tough one. We are in same boat - ds2 is 4 in May, but here all the children go to school in Sep when 4, so the nurseries/playgroups refuse to take them at that age, and are not set up to cater for them, plus I would lose his place at a 'good school' (Have you checked whether this is the case for you?) So I feel I have no choice. Ds2 will cope, but will be very tired.
We moved house from a different system, so ds1 didn't start school until he was 5 and 2 months (July birthday). Educationally he certainly benefited from having time one-to-one with me (3 mornings a week), and was so not ready for school at 4. However, he did really struggle to fit in at school when he went straight into Yr 1 (we moved so he was with children who'd been at sch for a yr already - as your ds will). In reception they do learn a lot about 'school stuff' - playground behaviour, what to do in assembly, walking round school in single file, etc.
I keep agonising over this, but I feel I have no choice, and ds2 IS different from ds1, and I think he will cope OK. If it had been ds1 (who is rather highly-strung), I would be agonising even more. The reception at our school is fairly relaxed - they do respect that some of the children are very young, and the afternoons are pretty much free play.
In our old county a friend had her girl in nursery until school, and the nursery staff were NNEB rather than teachers, and she certainly felt that her dd was ready for a bit more input on that side of things during the last 6 months ...
Bit garbled and incoherent, but I I hope that helps you!