Firing shots were heard inside the barrack of 18th Battalion CRPF at Kulgam town, 75 km from here, at around 10.45 PM on Saturday night, a senior police officer said.
Head Constables Suman Pillay and P Sibbu died on the spot while SD Murthy and Javed Hussain (same rank) were critically injured in the firing incident.
Hussain, who had sustained injuries from bullets and a sharp-edged weapon, later succumbed while being shifted to a hospital here, the officer said.
All the four jawans were posted as drivers in the force.
Superintendent of Police Kulgam Maqsood-uz-Zaman said the other jawans, who were present inside the barrack at the time of incident, have been detained for questioning.
"We have started investigations. It is premature to comment on it yet," he added.
This is the first incident of fratricide in CRPF this year in the Kashmir Valley.