Why is my daughter running a high fever and has been on an antibiotic for 4 days now?

Depends. If her fevers go away with meds and she otherwise looks ok, it is probably viral. Viral infections do not go away with antibiotics at all. They just have to "run it's course". If fever is not decreasing by day five or if your child looks bad even when not having a fever, check with your doctor. There are other things that could be causing it as well.
May be viral. Fever is the body's response to being sick. Fever activates the immune system and helps the body fight off an infection. Both viral and bacterial infections can trigger a fever response. Antibiotics kill bacteria, but do not affect viruses. In your daughter's case, she may have an underlying viral infection which needs to run it's course. However, if the fever continues, have her rechecked.