It happened just before 5 p.m. on the 4200 block of Clarissa Street inside a park in the city's Tioga-Nicetown neighborhood.
Police say the victims include a 22-year-old, a 36-year-old and two 26-year-olds. All four victims were placed in stable condition at an area hospital.
Authorities have not said what sparked the attack.
Officers have made an arrest in the case.
Anyone with information on this incident is asked to call police at 215-686-TIPS.