Four persons were killed and two others critically injured after a boiler in an aluminium factory exploded on Tuesday in West Bengal's Howrah district, police said.The incident happened at an aluminium plant in the Dhulagarh industrial complex."Four persons have been killed and two others have been injured. The victims were working near the boiler when it exploded. Investigation is on to find the reason of the explosion," said Superintendent of Police, Howrah, Sukesh Kumar Jain.