The blaze was reported from the factory located in Mongolpuri at around 3:30 AM and fire tenders were rushed to the spot.
The fire gutted three floors of the factory and rescue workers found five persons inside the debris.
All of them were rushed to Sanjay Gandhi Hospital where the four -- aged between 13 to 25 years -- were declared brought dead. 19-year-old Mithilesh was critically injured and undergoing treatment.
Investigators said the labourers died due to suffocation as they could not escape from the building because of an electric wire hanging outside it.
A case has been registered and investigations are on to ascertain the cause of fire.