The accident occurred when the bus with a total of 19 persons rolled about 30 feet below the road, some kilometres before reaching Phek town.
While three persons died on the spot, one succumbed to injuries on way to hospital, the police said.
The driver was among those killed.
Of the 15 injured, six were critically injured when the community bus in which they were travelling from Ketsapo village to Phek town met with the accident.
Six injured persons are undergoing treatment in different hospitals in the state capital.