Kumar had entered the tank and when he did not return for a long time, his brother Saravanan and relative Velmurugan went to check but they also did not come out, they said.
Subsequently, an employee of the hotel also entered the tank and met with the same fate, prompting his colleagues to alert the police, who found all the four dead.
Police said the victims died after inhaling some suspected poisonous gas.
Bodies of the four persons were retrieved with the help of Fire and Rescue Services Department and sent for post- mortem in a government hospital, police said.