Two of the four had serious injuries, the other two were moderately injured.
“Traffic management is in place,” a police spokesperson said.
“Motorists are advised to avoid the area if possible.”
Flaxton is 20km north of Christchurch and has been subject to a number of serious car crashes in recent years.
Two people were seriously injured in a crash involving three cars on Lineside and Mulcocks Rd in October 2019.
In 2014, another two drivers were seriously injured in a collision between a car and a four-wheel drive, while a year earlier a person was killed on Flaxton Rd after the car flipped and caught on fire.