The toll in the fire incident at an Oil India Ltd (OIL) facility in Assam's Dibrugarh district increased to four, with another injured worker succumbing on Sunday, an official said.
Three others who were also injured in the blaze on Friday are undergoing treatment in a critical condition.
One person was killed in the fire on the day of the incident, while two died the next day.
A mechanical helper, Sailen Gogoi, who was undergoing treatment at a private hospital here, died during the day, taking the toll to four, the official said.
All the victims were employees of Spectron Engineering Private Limited, a Mumbai-based company engaged by OIL.
A fire had broken out on November 6 after an explosion during maintenance work at OIL's intermediate tank farm in Tengakhat, the public sector undertaking said in a statement.
An intermediate tank farm is a facility where petroleum products are temporarily stored in tanks before being transferred for further processing or transportation.
The PSU has announced an ex gratia of Rs 10 lakh to the next of kin of each deceased, and Rs 5 lakh to each injured person.
Union minister Sarbananda Sonowal and Chief Minister Himanta Biswa Sarma were closely monitoring the situation, the PSU said on Saturday.
Sonowal, who is the Lok Sabha MP from Dibrugarh, called OIL CMD Ranjit Rath to take stock of the situation.
Sarma has directed his principal secretary to be in touch with OIL chairman-cum-managing director to ensure the best possible medical care for the injured and adequate compensation for the families of those killed.
(With inputs from PTI)