Two people were still being treated at the scene as of 2.45pm.
St John confirmed just after 3pm that the two other patients were also being transported to hospital.
“In addition to the two patients we have already transported, we have transported two further patients to Waikato in critical condition, one via helicopter and one via road,” a St John spokesperson said.
The highway is closed and traffic is being detoured south of Mangarino and Te Kumi Station Rds.