Four persons were killed and four others critically injured in a blast in a plastic factory in West Bengals Malda district on Thursday, police said. The incident took place in Sujapur area around 11 am, a senior police officer said. Four factory workers were killed and four others critically injured in the explosion that ripped the establishment apart, he said. A huge police contingent has been sent to the spot to bring the situation under control. Fire tenders are trying to douse the blaze and rescue survivors from beneath the debris, he said, adding an investigation is underway.