The mishap occurred last night when the driver lost control over the vehicle, they said, adding at least 14 passengers also sustained injuries in the mishap and they are being treated in Dhemaji and Dibrugarh in Assam.
The deceased were identified as Ganesh Gupta, Niyapee Nath, Anawari Katori and Sabnam Katori.
The bus was on its way from Silapathar to Yingkiong in Upper Siang district via Aalo in West Siang district.
Meanwhile, Yingkiong MLA Alo Libang visited the spot early this morning and announced financial aid for the victims.