The mishap took place near the Chandrain Panchayat building when the tractor, carrying about 40 members of a marriage party, including the bride and the groom lost control and overturned, Sadar Sub-Divisional Officer (SDO) Rakesh Kumar Singh said.
The four boys were identified as Chhotelal Sada (14), Markand Sada (12), Dularchand Sada (16) and Sujit Sada (12), brother of the bride, died on the spot, while eight others sustained injuries, he said.
The ill-fated tractor was returning to Darbhanga after the marriage in Veerganj village under Mohanpur panchayat in Saharsa district, the SDO said.