The arrested have been identified as Karbi Peoples Liberation Tigers (KPLT) former militants Kissan Rongpi alias Atur Hanse and Ram Teron alias Kirim Teron, United Peoples Democratic Solidarity (UPDS) former cadre Oliver Ingti and Nanda Das, Superintendent of Police Mugdha Jyoti Mahanta said.
A 9 mm pistol allegedly used in the killing was recovered with five rounds from the arrested persons, Mahanta said.
The four in their confessional statement claimed that they were engaged by the teacher's wife Suman Kumari Devi and her paramour Nanda Das for Rs 1,50,000, which she handed over to them, the day after the killing, the SP said.
Singh was an assistant teacher at Diphu Adarsha Hindi School and is survived by wife and two children.