The mishap took place at Clacso industrial unit in Bari Brahamana belt of Jammu.
While three labourers died on the spot, one succumbed to her injuries during treatment at GMC hospital here, officials said.
The deceased have been identified as Ganga Bhai and Savitri of Chhattisgarh and Pramila of Madhya Pradesh. The identity of the fourth labourer is yet to be ascertained.
A case has been registered, officials said, adding a police team has commenced recording statements of the industrial unit's owner and others.