The gunmen fired indiscriminately at men working in a scrap market on Sarki Road in Quetta, the capital of Balochistan province.
Four Shia men were killed instantly, local residents and a Shia organisation said.
The gunmen, who were riding a motorcycle, escaped before police cordoned off the area and started a search.
No group claimed responsibility for the attack though such incidents are usually blamed on the banned Lashkar-e-Jhangvi, a notorious sectarian group.
