The four suspects, whose identities have been withheld, were reportedly the ones who got in touch with gunman Aniceto Dejeto Sr. before he carried out his mission to kill Brillantes.
Dejeto, a barangay tanod in Sto. Tomas, Davao del Norte, was said to be a gun-for-hire.
Brillantes assassination was believed linked to the long-drawn conflict at the gold mining site at Mt. Diwalwal. The slain mayor was the president of JB Management Corp. that had a huge interest in the mining site.
The former lawman used to be a member of the Monkayo police until he was discharged from the service in 1990. He spent time at the Davao Penal Colony and was released only last April.
Dejeto was fetched from Sto. Tomas last Saturday afternoon by a Mitsubishi van, reportedly owned by a woman associated with the former policeman.
Dejeto, whose body remains unclaimed, was said to have been promised P1 million to do the job. He reportedly gave his wife P3,000 before he left.