Avon and Somerset Police said a 19-year-old man has been arrested on suspicion of causing death by dangerous driving and manslaughter by gross negligence.
A 28-year-old man has also been arrested on suspicion of causing manslaughter by gross negligence.
Their arrests follow the deaths of four-year-old Mitzi Steady from Bath, Robert Parker aged 59 from Cwmbran, Phillip Allan aged 52 from Swansea and Stephen Vaughan aged 34 from Swansea.
The girl’s grandmother who was also injured in the incident and remains in a critical condition in Southmead Hospital.