Superintendent of Police Satyaprakash said the CISF personnel Balbir was apparently angry over leave issues and fired at his colleagues from his service rifle, killing two of them on the spot at the company campus.
Two others injured by him in the incident were declared brought dead by doctors of Narayan Medical College and Hospital in neighbouring Rohtas district where they were rushed, the SP said.
The incident was reported at about 12:30 AM.
The accused CISF personnel, who hails from Aligarh in Uttar Pradesh, has been arrested, Satyaprakash said, adding senior police officials have rushed to the spot to probe the incident.
The four CISF personnel who were killed have been identified as Bachcha Sharma, N Mishra, Arvind Kumar and G S Ram, he said.
Officials said, "Preliminary information states that Balveer opened fire on his colleagues, using a service rifle, in an alleged fratricide incident".
Balveer allegedly used his INSAS rifle to shoot his colleagues who had assembled to leave for the shift change and after the first hit, he was overpowered by others present around.
It is understood that Balveer had come back after a two-month yoga course and had some issues related to leave, they said.
The CISF has ordered a Court of Inquiry into the incident.
CISF has been deployed for security at the under-construction thermal power project, a joint venture of NTPC and Bihar government.