Very normal! Four and five year old girls are evil!
Well... okay, not evil.

How about - they're busy exploring and testing the bounds of human social interaction?
Anyway, I had to come down hard on my dd about this kind of thing, too. She was both the instigator and the victim at different times. Just work with the teacher, talk to your dd at home, go over her day with her, and emphasize how important it is to think about how *everyone* feels. Is anyone sad? What can you do to make it better? How can you stand up for yourself when someone is being mean to you? How can you stand up for someone else?
Work on her empathy skills. As my husband used to say to my dd, "Are you using your powers for good?"
One of two things happens, as far as I've seen. Your dd develops a conscience and all is well. Or... she doesn't and she turns into one of *those* girls in high school. Given that you're already concerned and supporting the teacher, I don't think there's much chance of the second outcome. Your dd will be a very sweet girl by the time she's seven!