The ill-fated vehicle bound for Rumjatar from Barnalu in Okhaldhunga district drifted off the road and plunged into the gorge, police said.
The tractor driver Nima Sherpa and a 15-year-old boy were among the dead.
Three of the deceased died on the spot while the driver breathed his last at district hospital, police added.
According to preliminary investigation, the accident was caused by overspeed.
A man, who sustained injuries in the accident, was admitted to the district hospital for treatment.