As a parent of 2 boys and one girl, to me the differences are huge. Having studied developmental psychology in reasonable depth, and based on everything I have read and on my own dcs, it is clear to me that boys are not neurologically and socially ready for formal schooling at 4.
Some children are only just 4 when they have to go into reception.
Boys'fine motor skills are less developed at this age than girls, and they do often struggle with reading and writing.
My boys are fortunate that they are both exceptionally bright, and were generally well behaved, but they still found formal schooling and being in a very confined space very difficult. Dd, OTOH, found it perfectly acceptable, and she was quite a tomboy, being the youngest.
The education system in this country is designed for, and run mainly by, females, and I think boys get a pretty raw deal. They get forced into an unsuitable environment, then blamed and villified when they don't cope.
Sorry about the rant, if you've got this far, but this is a subject close to my heart. (And my dss are off to university this year and next, so I really have no axe to grind.)
Read Steve Biddulph - he puts it much better than me.