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.
India today assured all help to the Indian couple arrested in Norway for allegedly trying to discipline their 7-year-old son for wetting his pants.