The prime accused, Munna Dhali, an employee in the cable television business office of the family, and his three accomplices were rounded up from two areas on the city's outskirts, police commissioner Ranjit Pachnanda told mediapersons here.
Behala is a locality in south Kolkata, where the incident took place.
The four were presented in a lower court which remanded them to police custody till Sep 21.
The body of house owner octogenarian Gouri Bhattacharya and her son Dipak were found lying in a pool of blood on the first floor of the house. The bodies of the domestic help and an attendant were lying on the ground floor.
"The prime motive was dacoity. Munna had led his three accomplices to the residence of the Bhattacharyas as he was under the impression that the family had stacked at least Rs.5 lakh in the house," said Pachnanda.
He said Munna was known to the maid as he used to collect cable subscriptions and hand it over to the owner of the cable business, Gouri Bhattacharya, regularly. "The maid opened the door for the accused. They first killed the maid and the attendant and then went upstairs.
"Dipak was the first target, and then they killed Gouri," he said.
Besides, Munna, the three others arrested were Sattar Mandal, Naba Dhali and Rajesh Das.
"Munna knew Rajesh. The other two stayed in Munna's locality," the police commissioner said.
He said after killing the four, the miscreants were rummaging through the almirah, but got cold feet when the land phone suddenly rang. They then fled.
A police source said while interrogation of the relatives and associates of the Bhattacharyas did not yield any trail to the murder, the fact that Munna went to the family's cable television office two hours later than usual caught the attention of investigators.
"We found contradictions in Munna's statements. On sustained interrogation, he broke down and confessed to the crime. The three others have also confessed their involvement," he said.