Unidentified assailants opened fire on the workers' car with automatic weapons and rocket-propelled grenades Monday, killing the Americans who were trying to find a way to provide clean water in the northern city of Mosul.
A fifth person was in critical condition, the Virginia-based Southern Baptist International Mission Board reported.
Officials from the group said Tuesday that they have no immediate plans to pull their workers out of Iraq.
"Certainly we're concerned about safety. We will continue to evaluate the situation in every location we work in," said Clyde Meador, the board's executive vice president, at a news conference in Richmond, Va.
The group described the four as workers answering a call from God to serve through humanitarian work, rather than missionaries in the traditional sense, according to the Associated Press.
Witnesses told news agencies the missionaries had been driving through Mosul in a civilian vehicle without a military escort.
In a separate incident, two Western engineers and two Iraqis were killed Tuesday in an attack near Hilla, a town south of Baghdad.
The Dutch government said one of the engineers was from the Netherlands, the first of its citizens killed in Iraq since the start of the war. Police in Hilla had reported earlier that both foreign victims were German. The German embassy declined to comment, according to Reuters.
Gunmen also killed three Iraqi policemen in Mosul Tuesday afternoon in a drive-by shooting, police at the scene said.
The incident in Hilla brought to eight the number of civilians killed in as many days, in an apparent trend toward targeting foreign civilians working in Iraq.
On March 9, two U.S. civilians working for the Defense Department and their Iraqi translator were shot dead in a road ambush on the road between Hilla and Karbala. They were the first American employees of the U.S.-led civilian administration to be killed in Iraq.
The U.S. military said six people had been detained over the incident and four were believed to be Iraqi police.
The past week has also seen a spike in attacks on U.S. soldiers -- roadside bomb blasts in and near Baghdad have killed nine since Wednesday.