Those convicted on Tuesday are Md Saidul Islam, Md Al Amin, Abdur Rashid and Rafiqul Islam. They hail from the same Chaklengura village.
According to the prosecution, the convicts abducted Naluaparha Government Primary School's grade-I student 'Emon' on Aug 19 last year.
They demanded Tk 100,000 and one bhori of gold as ransom, which Emon's family could not pay.
For not getting the ransom, the men took the boy to Latapata hill near the Durgapur border and killed him.
Emon's mother filed a case over the kidnapping a day later. Police pressed charges in court on Oct 22 the same year.