First off, the teachers and director of the Adventure Club should be dealing with the behavior as it occurs not just saying "she has an attitude problem." The actvities should be supervised at all times so why did no one intervene and provide consequences for the behavior of the 4th grader? I would keep a close eye on it and ask that the teachers be more involved.
I would not hesitate to contact the parents. I for one believe that parents should be more involved in what their kids are doing at school, at friends houses etc. If my daughter had done anything like that, I'd want to know about it so I could take appropriate action. Whether or not the parents do anything about it is up to them. Sometimes just calling someone on their behavior is enough to prevent further bullying. Sometimes not. But at least you know you did everything in your power to try to work it out.