First off, I am so sorry you have to deal with this. Four seems to be the age when mean behavior sets in. At least that has been my experience. My daughter just turned 5, and this past year at preschool we experienced a bully. Girls seem to be meaner than boys - they use words to hurt, and at this age, the kids are so literal, they take everything to heart.
The first thing I would do (and you may have already done this) is to contact the camp director. Most camps we've been involved with have teens or college students as counselors, so I'd skip on to the adult in charge. Explain the bullying and your daughter's medical condition. They should have a talk with the kids in question and lay down the law that teasing is not acceptable.
I would also arm your daughter with responses. You can start with things like, "it hurts my feelings when you..." which of course only works when the aggressor doesn't mean to hurt feelings. Sometimes it works, but if they meant their words to cause pain, it has no effect. Next, she can tell the kids, "you have very bad manners." This one seemed to work best as it tosses things back in the other kid's court. When my daughter used this one on the girl who was being mean, it kind of shocked her and she didn't know how to respond. The girl backed off. Another phrase to teach her is, "you may not speak to me like that." Again, it shocks the kid and they don't know how to respond.
Our situation was different, and with only only girl. She tried to push my kid around, and made her cry. Once my daughter knew she could respond (and knew what she could say) she felt empowered. In the end, she stopped playing with the "mean girl" and when she started ignoring the girl, the girl desperately tried to be friends. My daughter would have none of it! I think teaching her those phrases really helped her confidence. There have been no further incidents for us.
Hope it helps! Best of luck!