It happened around 7 p.m. at 69th Street and Buist Avenue in Southwest Philadelphia.
Police say a 28-year-old man who was an employee of the gas station was shot eight times throughout his body. He is currently listed in critical condition at an area hospital.
"He was still working and he was still inside of the garage when some sort of argument took place that escalated and led to the shooting," said Chief Inspector Scott Small.
Three other men were injured in the shooting. All three victims are listed in stable condition at this time.
Chopper 6 was over the scene as police cordoned off the entire area of the gas station.
Police say they found as many as 16 bullet casings, most of them right outside the garage. More casings were found behind the gas station on the 6900 block of Theodore Street.
"It appears there was some sort of shootout right at the garage door bays," added Small.
The quadruple shooting is the latest in the out of control gun violence running rampant across the city.
"Our communities are in trouble and we are working with our partners to try and figure out, number one, why? And number two, how we can help them, but we need their help as well," said Deputy Police Commissioner Robin Wimberly.
No arrests have been made at this time.
Anyone with any information is asked to call police at 215-686-TIPS.