A week after a devastating two-car crash on the Arborway in Jamaica Plain injured six people, four people remain in the hospital, while a 15-year-old boy and a man have been discharged, authorities said. The crash involved a Mazda SUV containing four youths and a Ford pickup truck containing two men. The 18-year-old driver of the Mazda remains in the hospital, as do two of the car’s passengers, ages 13 and 14. The man who was driving the truck also remains hospitalized. All are expected to survive, said Jake Wark, spokesman for Suffolk District Attorney Daniel F. Conley.