It happened just before 5 p.m. on the 4200 block of Clarissa Street inside a park in the city's Tioga-Nicetown neighborhood.
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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.
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Anyone with information on this incident is asked to call police at 215-686-TIPS.