Eight firefighting vessels were battling the pre-dawn blaze at the Abkatun A-Permanente platform on the Gulf of Mexico's Campeche Sound, the state-run firm said in a statement.
The blast took place at 3:40 am in the dehydration and pump area of the platform, located off the coast of eastern Campeche state, the ASEA energy safety agency said in a separate statement.
Some 300 workers were taken to other platforms in the area while the injured were transported to Pemex's general hospital in Ciudad de Carmen. Two were in serious condition.
AFP obtained a video taken from a boat showing huge flames and smoke engulfing much of the platform in the middle of the night.
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Mexican media published pictures of firefighting vessels battling the blaze with water cannons as a huge column of smoke went skyward.
"Regrettably, the death of four workers in the accident on the Abkatun A-Permanente platform has been confirmed," Pemex wrote on its Twitter account, raising an earlier toll of one dead contractor from the Cotemar company.
"The priority in this emergency is to protect the safety of personnel, contain the fire and care for the injured," the ASEA agency said, adding that all the workers were removed from the rig.