The victims had had words with another group of people shortly before the masked gunman approached the fast-food restaurant Wednesday, opened fire, then fled, the Chicago Sun-Times reported Thursday.
The victim, Markeyo Carr, died at the scene, the Cook County Medical Examiner's office said.
Also shot were a girl, estimated to be about 15 years old, and two 18-year-old males, police said.
"Two of the male victims have documented gang affiliations," Chicago police officer Veejay Zala said.
The Chicago Tribune said the shooting occurred in an area where the Latin Kings and La Raza street gangs have been feuding.
No arrest was immediately made.
Faye McCoy, a retired nurse, told the Sun-Times she heard the gunshots from across the street and went to the parking lot to see if she could help once the gunfire stopped.
"There was no help for him, he was shot in the temple it looked like, he took his last breath with me standing there," McCoy said. "Blood was everywhere."
McCoy said she recognized Carr from working with local youth groups to prevent violence and didn't think he was in a gang.