I'm obviously out of the cultural loop here.
4 year olds are playing in pre school here. Learning not to lump each other on the head with play dough. None of the staff were teaching the kids to read and as far as I remember none of the other mothers made any mention of teaching reading at home.
Learning to read begins at 6 when they start elementary school.
I'd be surprised if developmental readiness was reliably uniform at such a young age, because recently with the introduction of the option to fast track 5 year olds into first year ele there have been constant reports of ever higher numbers of children needing additional one on one support. There appears to be an issue because they are tending to lag and risk being left behind as the older children leap ahead with more complicated work.
Come to think of it, that first year, the learning to read was completely painless compared to all the other subjects. Certainly my average child was reading fluently by the end of the year. I did have kittens at the mid way point because there weren't any graded readers available either via the school, the library or the bookshops, but I suppose that was because there is no "sight word" issue in Italian. They just read books generally aimed at their age group.
I chose to delay teaching my son to read English until the Italian was "in", to avoid creating confusion. Happily it turned out to be a transferable skill and he taught himself by dint of reading my old ladybird books by torchlight under the duvet when he was supposed to be sleeping.
I nearly fell over when he brought a book down, asked "is this true mummy ?" and read the bit he wanted clarifying. Swiftly followed by removing jolly phonics from my amazon wish list. He was about 7 at the time.
The thought of leaving the acquisition of reading to an entirely organic process where solely the child dictates the timeline makes me feel a little faint, but at the same time I'm not sure I understand the need for a huge rush to get preschoolers reading.
Unless of course they are champing at the bit, in which case I think it is only right and proper that their inclinations be encouraged and supported.