The force force confirmed the four boys in custody are all from Liverpool and said they were arrested after site security staff called the police shortly after 4pm.
A 15-year-old boy from Dudley, West Midlands, was airlifted to the Royal Derby Hospital after suffering facial injuries.
His condition is described as stable.
The force has appealed for witnesses to contact police.
Alton Towers is one of the UK’s most popular theme park and a popular choice for end-of-year school trips.
It hit the headlines in 2015 when two carriages on rollercoaster the Smiler crashed leaving five people with serious injuries.
The ride reopened in March this year.