The soldiers were accused of forcing two Iraqis off a bridge into the Tigris river in January.
The Iraqis had initially been picked up for curfew violation near the Iraqi city of Samarra.
One of the two, Zaydun Maamun Fadhil al-Samarai, drowned while the other - his cousin Marwan al-Samarai - managed to get out of the river.
The case against the four soldiers comes in the wake of revelations about rampant abuse of prisoners in Iraq, Afghanistan and at Guantanamo Bay.
The US army said in a statement that it had brought charges against four soldiers from the 4th Infantry Division.
The drowning incident almost went untold until a popular online blog - healingiraq.blogspot.com - posted a public appeal from al-Samarai's mother calling for an investigation into her son's death.
She claimed the two cousins had been hauling toilet spare parts from Baghdad to Samarra when their truck broke down on 3 January.
After repairing it, they were stopped at a US checkpoint at 22:45, 15 minutes before the beginning of the nightly curfew.
The two Iraqis were allowed to pass but were then followed by a "mechanised vehicle". They were stopped, forced out of their truck, handcuffed and taken to a bridge overlooking the Tharthar Dam where they were then allegedly forced to jump into the Tigris river.
The mother wrote an open letter to US President George Bush and British Prime Minister Tony Blair pleading for an investigation into her son's death.
An investigation was opened by US occupation forces on 8 January.
In the dock
First Lt Jack Saville and Sgt 1st Class Tracy Perkins were charged on 7 June with manslaughter, assault, conspiracy, making false statements and obstruction of justice.
Two others were charged on Monday. Sgt Reggie Martinez was charged with manslaughter and making a false official statement and Special Terry Bowman was charged with assault and making a false official statement.
All four will face the military version of a grand jury proceeding before a decision is made to carry on with a court martial.
Army spokesman Lt Col Jonathan Withington said he did not know whether the soldiers pushed the Iraqis off the bridge or forced them to jump.
Meanwhile, a report by the US army's inspector general, Lt Gen Paul Mikolashek, criticised army policies on prisoner operations. Improper training of jailors and interrogators has been identified as a major shortcoming.