Two of them returned to the country on January 23 in the same flight as the person from Kerala declared as the first confirmed case of the dreaded disease in the country.
Two others, who came to the country from abroad after January 15, are also under observation as they have exhibited some classical symptoms of the viral disease like cough, cold and slight fever.
"We have kept under observations those who returned to the country on Janaury 23 with the first confirmed corononavirus patient in India. If need be, we will send their samples for testing," said hospital superintendent Ashish Manna.
One of them is a resident of the southern outskirts of the city and the other in North 24 Parganas.
"We have also made arrangements for testing of samples at the virology laboratory of ICMR within our hospital campus from today. So long, we were sending all the samples to the National Institute of Virology, Pune," he said.
The authorities said some people who have returned to the country from abroad with symptoms like cold are coming to the hospital fearing they could have contracted the disease.
The authorities have been assuring these people and asking them to remain in isolation at home.
The state health department is also trying to ascertain whether any person hailing from Bengal availed of the same flight by which the patient confirmed as the third case of coronavirus in India came to the country from Wuhan.
( With inputs from IANS )