The collision happened at the junction of Shaw Road and New Road in Dudley at around 7.50pm on Tuesday.
Three adults and a child were injured, including a female passenger who needed advanced trauma care.
A man who was driving one of the cars and a child who was a passenger in a parked car both suffered "potentially serious" injuries.
The fourth injured person, a woman who was behind the wheel of a parked car, was also taken to hospital but is not thought to have been seriously hurt.
Photos from the scene showed the cars had been left with major damage, with the doors removed from one of the vehicles involved.
Police officers and firefighters were sent to the scene along with paramedics and the road was closed while the scene was cleared.
A spokesman for West Midlands Ambulance Service said: "On arrival, we found four patients. The first, a woman, a passenger in one of the cars, was assessed and had sustained serious injuries, she received advanced trauma care on scene and was conveyed to Queen Elizabeth Hospital Birmingham for further treatment.
"The second, a man, the driver of one of the cars, was assessed and had sustained potentially serious injuries. The third, a child, a passenger in a stationary car, was assessed and had sustained potentially serious injuries.
"The fourth, the driver of the stationary car, a woman, was assessed and had injuries not believed to be serious. They were conveyed to Russells Hall Hospital for further assessment."
West Midlands Fire Service said the male driver was "complaining of chest pain" before being taken to hospital, and an occupant of the parked car had to be cut out.
"Our crews made all affected vehicles safe and closed the road due to a large amount of fuel having been spilt which made the surface slippery," the spokesperson added.
West Midlands Police said one of the female drivers sustained a serious leg injury, but it is not believed to be life-threatening.
Anyone with information is asked to get in touch via Live Chat on the force's website, quoting log 3542 of 15 September.