An air ambulance was called to the scene after the accident on the "Smiler" ride at Alton Towers, a park run by Merlin Entertainments Plc in Staffordshire, central England.
One car was empty, the other contained 16 people.
The Smiler, the latest new ride at Alton Towers, opened in May 2013 and is the world's first 14-loop rollercoaster, according to the attraction's website. It can reach speeds up to 85kmh.
"Of the 16 occupants on board the ride, two teenage boys and two teenage girls, have sustained serious leg injuries," said a spokeswoman for West Midlands Ambulance Service.
A platform had been built to allow emergency services to reach the occupants on the ride who were about 25 feet up in the air at an angle of about 45 degrees, she said.
"The 12 other occupants on the ride are said to have suffered minor injuries and it's hoped that they will be discharged from the scene," she added in a statement.
Merlin said in a statement there would be a full investigation once it had recovered the guests. The collision took place on the lower part of the ride, it added.
