Two men, a woman and a teenager were arrested Monday on suspicion of setting fire to a house in Chiba Prefecture and killing its sole resident.
The four friends were spotted near the house in Inzai when the fire broke out shortly after 4 p.m. Saturday, the police said. The house was completely incinerated.
The resident was Yoshiko Ebihara, 55, who lived alone. An autopsy will be carried out to confirm her identity.
The three adult suspects, all 20 years old, were identified as Junya Nakauchi of Tokyo's Sumida Ward, Taiga Kanazaki of Inzai and Miku Kanno, who has no fixed address. Kanno, a woman, is believed to have known Ebihara, according to the police.
The name of the teen, a 16-year-old girl from Tokyo, is being withheld as she is a minor.
The four were in a car parked at a rest stop in Fuji, Shizuoka Prefecture, when investigators located and arrested them.
A 69-year-old man living near Ebihara said he could not think of any reason for attacking her.
"She had some problems with her legs. She liked to talk and I can't think of any difficulties she might have had," he said.