Emergency services were called to the scene, near Newhouse, at about 09:00. Firefighters used hydraulic cutting gear to free those who were trapped.
Ambulances took a woman and two men to Wishaw General Hospital and a third man to Monklands Hospital.
The road was closed in both directions while emergency services dealt with the crash.
Four people have been injured in a crash between two cars on the A73 in North Lanarkshire.