Additional District and Sessions Judge Niranjan Singh also imposed a fine of Rs 10,000 on each of the convicts for the crime committed on March 31, 2007 at Karari Tintanga village in Bhagalpur district.
The four convicts were Buchi Mandal, Dilip Mandal, Yashoda Devi and Prerna Devi.
Public prosecutor Sri Kishore Jha said a young monk identified as Chandan Swami was strangulated to death.
The monk's uncle had registered an FIR accusing seven persons of committing the murder.
"Three of the accused were set free due to lack of evidence against them. The others were found to be guilty of committing the heinous crime," Jha added.