The BJP worker Pramod had suffered head injuries after he was hit with a brick allegedly by CPI(M) workers following a clash during a victory rally by CPI(M) workers at Idavilangu on May 19, the day Kerala assembly poll results were declared.
BJP had alleged that CPI(M) workers, who had taken part in the rally, had attacked Pramod, who succumbed to injuries at a private hospital here on May 20.
The four arrested are Siyad, Tutu, Madhu and Mithun, police said.