Local police told Mainichi Daily News the massive explosion occurred at the Nagoya oil-tank facility while a team of five workers were cleaning the No. 2 tank.
The workers had transferred most of the gasoline to another tank and were moving the remaining fuel to an oil tanker when vaporized gasoline suddenly burst into flames.
Three workers inside the tank were killed instantly. One worker some 30 feet away was also killed.
The blaze took firefighters nearly four hours to extinguish. Police said the injured workers suffered burns.
The Exxon Mobile facility is located about six miles from the Nagoya city center.