Officials say an argument broke out at about 9 p.m. Saturday near the vigil in the Elmwood section of the city, and shots were fired.
Three youths and a teenager girl were taken to Children's Hospital of Philadelphia, where they were reported in stable condition. A male suspect was arrested.
The vigil was being held for a 16-year-old boy struck by a privately owned garbage truck as he was riding his bicycle in the area Friday. No charges have been filed in the case.