The incident took place near Chhapraili village on Purnia-Sonali road when the driver lost control control of the vehicle, rammed into a tree and overturned on the labourers sitting there, DSP Rajkumar Sah said.
Four of them were killed on the spot, while four others suffered injuries, he said.
The deceased have been identified as Virendra Uraon (30), Manok Uraon (25), Anil Uraon (20) and Jitendra Uraon (40).
The bodies have been sent for post-mortem and the injured were shifted to Sadar hospital, Sah said.