The deceased are Jahangir Hossain, 35, Manjur Sheikh, 55, Rana Sheikh, 24, and Rubel Sheikh, 30, of Charkandi village in the upazila.
Sub-inspector of Katakhali Highway Police Station Moloy Roy said a bus crashed into a trolley carrying a group of construction workers at Berboalia on Dhaka-Mawa highway in Mollahat around 7:15pm on Thursday.
The accident left Rubel dead on the spot and 14 others injured.
The injured were first taken to Mollahat Upazila Health Complex from where some of them were sent to Khulna Medical College Hospital (KMCH).
Three of the injured succumbed to their injuries at the KMCH in the early hours of Friday, said the police officer.
He said they seized the bus after the accident but its driver and his helper managed to flee.