Actually, the mean girls start even earlier than fourth grade unless the school is very well run. Usually they start targeting a few children in the second half of second grade, pick up in the second half of fifth grade and reach a crescendo in seventh grade, particularly if these children are more independent.
Don't put up with it, and don't let the school put up with it, either. First, get permission to observe the classroom and the playground quietly for a morning, to make sure your daughter is right, then ask for a meeting with the principal and the teacher to tell them what's going on because you know that they wouldn't want to permit any behavior that reflects so badly on them and on the school, and also because it hurts your daughter's feelings.
You also should encourage your daughter to wear clothes that don't invite teasing (i.e., the same kind of clothes that everyone else wears) and to be friendly with them and to every child at school, but never to grovel. Bullies don't pick on strong children.