The four -- three Thais and one Vietnamese -- were found at the bottom of the tank at 2:30 p.m., according to the North Gyeongsang Province Fire Service Headquarters.
The workers, whose names have yet to be made public, are believed to have suffocated while doing maintenance work inside the tank at the factory in Chuksan Port in Yeongdeok, some 250 kilometers southeast of Seoul.
The 42-year-old and 28-year-old Thai workers and the 53-year-old Vietnamese person died in the accident, and the other victim, a 34-year-old Thai, is currently in a coma, the fire headquarters said.
The tank stores byproducts from the processing of fish and shellfish, it added.
The rescue authorities said the workers appeared to have entered the tank without safety equipments even though there was a high risk of inhaling toxic gas from decomposed fishery products.
Local police said they're investigating company officials to find out whether they followed established safety regulations. (Yonhap)