Have just read the whole thread and I really think they should be better looked after. It is the schools responsibilty to look after them.
Lots of 4 year old have a smaller playground with only young children in it until they get older. Incredible that they are pussy footing around asking everyone what happened - the staff should know what happened because they should have seen it.
My son was being picked on by another lad in his class, he was 5 at the time, he didnt want to go to school that morning. I didnt have time to go in as I was working but I rang the headteacher and told her. She asked the name of the boy (who apparently was fairly well known for bad behaviour - I found out later) she assured me she would sort it out and it would not happen again. And she did - just like that - never happened again. How did she do it - called both children into her office, told them that she did not want to see Child A playing any where near my son in the playground at all, and if he did, then she told me son, not to tell the dinner ladies, or the playground teacher, but to come straight to her office and tell her - the head mistress - so that was the end of that. My little boy was amazed that the head, Mrs Drake, actually had seen what had happened.