The court also sentenced the convicts to three years in jail and fined Tk100,000 each in another charge.
The four who were give death penalty are Shahidul Islam Chowkidar, Barek Chowkidar, Bashir Matubbar and Moazzem.
People who have been jailed for life are Abdur Rob Chowkidar and Mozibur.
Additional District and Sessions Judge Muhammad Abu Taher announced the verdict after cross-checking 19 witnesses and examining documents.
According to the prosecution, Naya Mia was thrashed to death on April 10, 2012 when he was returning home at Cholavanga village. The murderers also snatched Tk70,000 from him.
Md Nasir Uddin, son of Naya Mia, a resident of Chalbhanga village under Amtali upzila, filed a case with Amtali police station accusing the six people.
After the announcement of the verdict, Nasir said: “My father has been killed by the hoodlums. We are happy with the verdict. We are also demanding speedy execution of the verdict.
Defence counsel Kamal Kanti Das, however, said they were not happy with the verdict and would challenge the conviction in High Court.