I have a 4 1/2 year old son who has been having a behavioral regression, like your daughter. I honestly don't know what to tell you, I have been trying to get his behavior/emotions in check for a few months now. At home he is getting better, but like you and your daughter, school is an issue. He will be going to kindergarten next year so I worry that if we don't get his behavior regulated soon, he'll have problems there also. I have really been taking a look at myself and my husband, and the way we 'deal' with him. I've been trying to keep myself calm when he has his tantrums, and really talk to him about his behavior. I try to say things like "Is that the way to get what you want?" or "Could we maybe have done something different?" I try to get him to look at his behavior and recognize the problem, and then we fix it together. I don't really know what else to do. I know a lot of this is emotional so I try to be encouraging, and like you, get him to calmly state his frustrations or disagreement rather than have a melt down.
I realize this wasn't much of an answer. Just letting you know that you are not alone, and if you find any magical fix, Please forward on!
~Katie