The explosion occurred in Malir area of Karachi at a unit that makes cement blocks used in constructing houses.
One of the four injured men was in a critical condition, officials at Jinnah Hospital said.
Rescue teams took the injured to hospital while a bomb disposal squad swept the area.
Police officials said the bomb was apparently planted yesterday to target a Shia procession.
Security forces cordoned off the area and prevented the public and media from approaching the site of the blast.