Three teenagers and one man riding The Smiler at Alton Towers sustained serious leg injuries when their carriage slammed into an empty one on the £18m ($27.5m, €24.7m) ride.
Paramedics are still currently treating them on the ride - almost three hours since the crash - as they have not been released. They will be airlifted to hospital once they are free.
The victims are an 18 and 27-year-old man, and two females aged 19 and 17.
West Midlands Ambulance Service was called at 2.09pm to the Staffordshire theme park to a report of a collision on the 52mph ride.
Community first responders based at the theme park were first on scene. Four ambulances, three Midlands Air Ambulances from Strensham, Cosford and Tatenhill, the Warwickshire and Northamptonshire Air Ambulance, a MERIT trauma doctor and three BASICS doctors, paramedics from the Trusts Hazardous Area Response Team (HART) and several senior paramedic managers are currently in attendance.
A West Midlands Ambulance Service spokeswoman said: "Of the 16 occupants on board the ride, two teenage boys and two teenage girls, have sustained serious leg injuries.
"A platform has been build up in order for emergency services to reach the occupants on the ride who are approximately 25ft up in the air at an angle of about 45 degrees.
"Our HART paramedics, doctors, Staffordshire Fire and Rescue Service and Alton Towers' rope rescue staff are working at height to carefully release and treat the four seriously injured teenagers. 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.
"Once released from the ride, all four seriously injured teenagers will be airlifted to major trauma centres at University Hospital Coventry and Warwickshire and Royal Stoke University Hospital for further emergency treatment."