Drug trader Jewel was detained with 100g heroin while Alamgir Hossain, 20, Shahed Ali, 21, and Sumon Miah, 32, were arrested with a pistol, a revolver and three rounds of bullet.
DB Joint Commissioner Abdul Batan disclosed it at a press briefing in its media centre on Thursday.
He said the three fetched the firearms from Jessore to commit crimes like extortion and to establish dominance in Saydabad area. They were involved in criminal activities for long.
Legal action would be taken against the four, he also said.