Ok, well, voice of dissent here, I think
Personally, I think 4 is pretty young to be at school full time (bearing in mind that usually the more staggered intakes are for the younger kids). In fact, I'm seriously wondering about leaving ds at nursery for an extra term rather than putting him into school next Sept.
I think that shoving kids who aren't phyically ready into full time school is a bit pointless. Not that I'm saying anyone here is doing this, and obviously mums know whether their individual kid can cope with it ... but how is the school meant to know?
Also, from the teachers POV, drip feeding presumably gives them more time to settle those who need it/get to know indvidual personalites eyc.
My mum was a reception school teacher, and she said that she has at least one kid in every intake who really still needs a nap after lunch becuase they find school so knackering. In fact, she has mats set up in one corner so kids can have a snooze if they need one. She would also have a quiet word with mums & tell them to take kids off, if practical, when she felt they were getting tired.
I can see that it is an ungodly PITA (I work pt from home so have less hassles than about 99% world on this one, have to confess), but I can see why its done too.