Why might my 4 year olds left knee be swollen, hot, and red? No injury that I know of. She did get stung by something yesterday but that is on her arm

Arthritis or trauma. it is possible arthritis or trauma so see her pediatric doctor for diagnosis. .
See your doctor. Hard to tell without a physical exam, some lab tests and a possible x-Ray and/MRI or CT to see if synovitis, septic joint, or JRA - a much more chronic and serious disease. Please see your primary pediatrician right away.
Painful? If it's painful it could be an infection of the knee joint which can be dangerous. Please take her to the pediatrician. .