4 year old with temp of 102.3 and lower back pain. what could be wrong?

Several things...: Several things could be wrong, but with a fever that high in a 1 year old, you should be checked out by a physician without delay. Call your physician on call or go to the er/urgent care.

See a pediatrician. : Right away 4 year olds should never have back pain especially with a fever. Could be benign like the flu. But could also be a kidney infection, of spine abscess.

Viral infection: Is on the top of my list. In case the child continues to have a fever over 3 days or noticeable change in child's general well being occurs, seek a medical attention.