The bodies of the three men and one woman had bites and scratches presumably made by "a large animal," Kazuno Police Station spokesman Noboru Abukawa said. He added that police heard roars of a bear or bears and saw bears from a helicopter while searching for the victims.
The victims, who were aged 65 to 79, were killed in a mountainous area within a 2.5-kilometre radius since May 20.
On Friday, hunters shot and killed a female Asiatic black bear with a body length of 1.3 metres about 20 meters southwest of where the fourth victim was found dead, but Abukawa said it was unclear whether the dead bear had attacked the victims.
Police are enhancing patrols and have distributed fliers alerting residents around the mountainous area to exercise caution.
Nationally, 168 bear attacks have been reported between April 2014 and March 2016, according to Environment Ministry data, and 176 people suffered injuries or died in those attacks.