The robberies occurred on Sept. 15, 24, 27 and 28, the Chicago Tribune reported.
In all of the cases, the suspect pulled up next to the victims in a silver sports utility vehicle, told them he was a federal officer, and ordered them to get into the vehicle.
Then, the suspect showed all of the victims except one a gun and demanded cash, police said. None of the victims was harmed.
The suspect was described as a black man in his 40s, about 5 feet 8 inches tall and weighing about 170 pounds.