The four injured have been given medical care and are being treated while the attackers are yet to be identified.
Yesterday, a Communist Party of India (Marxist) worker was injured after a crude bomb was hurled at a rally.
The incident happened a day after Mayor V K Prasant was attacked by Bharatiya Janata Party (BJP) workers at the city corporation council.
Meanwhile, CPI(M) District Secretary Anavoor Nagappan alleged that the BJP workers had unleashed violence on the workers of his party during a procession.
In October, a RSS activist was injured, while bombs were hurled at a BJP party office.