My 5 year old is running a fever of 104.0. Should I take him to the er?

Maybe. It depends upon what other symptoms may be present, whether they are talking fluids effectively and eating ok and they are acting normally. You can try to bring down the fever with children's Aspirin or tylenol (acetaminophen) or bathing them in tepid water, or washing them down with a cool washcloth. If it gets much higher, would consider having them seen by a doctor.