Pol Lt Tipol Wadsungnoen, a duty officer of Wang Khon Daeng police station in Nadi district, said the accident occurred at 2am between kilometre markers 45 and 46 on Highway 304 (Nakhon Ratchasima-Nadi road) when the trailer truck, fully loaded with cassava root, rammed into three vans, a Toyota Yaris car, one Toyota Vigo pickup and a motorcycle which were going uphill.
The 18-wheel truck then plunged down a cliff and hit a tree 30-50 metres below, leaving the six other vehicles badly damaged at the roadside.
Altogether four people were killed and 16 others injured in the accident.
The injured were admitted to Nadi and Kabin Buri district hospitals.
Two of those killed, both male, were identified as Chaipat Lathikul, 42, of Buri Ram's Nang Rong district, and Prayoon Srikhamthong, 32, of Non Sang district of Nong Bua Lamphu province.
The two others killed were a man and a woman, who were not immediately identified.
According to an account from a van driver, the truck was believed to have lost its brakes on the controversial highway, which cuts through the Khao Yai wildlife zone.