The anti-Qaeda fighters, also known as the Sahwa, have been frequently targeted by gunmen and bomb attacks due to the group's collaboration and work with the United States since 2006. 
In Sunday's attack, medical officials told AFP that at least three other fighters were also wounded in addition to the four killed. It is not yet clear who is responsible for the attacks, but Sunni Muslim militants have frequently targeted the Sahwa due to their affiliation with the U.S.
In related news from Iraq Sunday, at least two people were killed and an additional six wounded after a roadside bomb exploded in Baghdad's Jihad area. 
More than 6,750 people have been killed due to violence in Iraq in 2013 alone, according to AFP figures.