I am not sure that electronic devices and a lack of real activities are the only reason not all 4yos are not ready for school.
My nephews and nieces have grown up in Sweden where there is far less health and safety and far more hands-on activities for children. There you will frequently see a 4yo wielding a hammer or stirring the contents of a frying pan. Nurseries make sure children lend a hand in preparing their own meals, and there is plentiful exposure to outdoor activities. But still everybody I know there would be horrified at the idea of sending a 4yo to school. Not because they feel they have failed as parents but because their 4yos are....errr...4yos.
I often compare my ds with his cousin. Both of them were late developers, but ds rather earlier on all counts: language development, manual control, social skills etc.
His cousin started school at 6 when he had grown out of all the toddler traits. He thinks of himself as rather a bright boy because he coped so well with school at 6. This confidence means he works hard and is making very good progress.
Ds started at 4 at an age where a lot of his development was not finished (though more advanced than his cousin at that age). By the end of the first term he was already telling us that "I'm dim", "I can't do the things the teacher wants me to". His self-esteem has never recovered and he still thinks of himself as dim.
He would have been a lot better off playing in the snow and making biscuits at 4.