Four year old girls are bullies. And victims of bullies. And it's not just "the way girls act" and the girls who are threatened emotionally and/or psychologically - which is the preferred methodology among girls, as opposed to threats of physical violence - are not to blame. Teachers who ignore this problem or deny that it is a problem need to change their ways of thinking and acting.
I'm doing a good bit of reading up on the topic, and recognizing and identifying some baggage from my past in the process. I'll try to report more once I finish reading Odd Girl Out, which, by the way, is quite a fascinating book.