They were identified as Anil Chakma, Chowpinto Thaman, Sanjiv Singh and Arvind Singh, the police said today.
Another six accused who are still at large includes Bishnu Chakma, Boksu Chakma, Patharmoni Chakma, Dayalu Chakma and Mytiodhan Chakma.
During the course of interrogation Anil Chakma admitted to collecting money from Bijoypur are and depositing the amount to Bishnu Chakma, sources said.
The ransom letter was written on UPDF pad and signed by its commander Sumana Munglang, and its 'finance minister' R Gohain.
The police recovered 8 mobile handsets, daggers and one black veil from their possession.