The accident occurred on National Highway 37 at Amguri in Koliabor when the mini bus travelling from Nagaon to Jorhat collided with the truck coming from the opposite direction.
Three passengers in the bus lost their lives on the spot while the fourth died at Jakhalabandha Hospital where the seriously injured were admitted, police said.
Police has begun probing into the accident and the bodies sent for postmortem examination and identification, the sources added.