The policemen had taken a break from the patrolling duty and were having tea at a small restaurant when the unidentified gunmen riding on motorcycles opened fire on them in Karachi's Korangi area.
Three constables and an assistant sub-inspector were killed on the spot, police said.
After the firing, the attackers took away the policemen's official weapons.
Today's incident was the first target killing of policemen in Karachi after a lull of around two months since the paramilitary rangers intensified their clean-up operations in the city.