Five men had gone down to clean the tank in a household in the area but did not come out for long. They were later pulled out in an unconscious state by fire department personnel after an hour-long operation at around 10 in the morning.
It is suspected that they inhaled toxic gases inside the tank.
Three of them were taken to the Fortis Hospital, while the other two were taken to the AIIMS Trauma Centre and the Safdarjung Hospital.
Four workers, identified as Swarn Singh (45), Dipu (28), Anil (23) and Balwinder (32), were declared brought dead.
Jaspal, Swarn Singh's son, is currently undergoing treatment at Fortis.
All five are residents of Ambedkar Colony in Chhattarpur.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)