Officials said a country boat carrying about 60 passengers, mostly daily labourers, capsized after it left Dharampur bank towards the char (riverine) island at 7.30 AM.
Four bodies, including that of a woman, have been fished out by the rescuers who are mostly locals, while eight persons were rescued.
Nearly 40 others are still missing.
Senior district and police officials are camping on the spot and supervising rescue operations.