Spc. Steven Ribordy, 25, of Salina, Kansas, also will receive a bad conduct discharge from the Army as part of a plea deal. He also agreed to testify against other members of his unit.
The prosecutor, Capt. John Merriam, had pressed for the maximum five years in prison.
"The execution of prisoners is arguably the greatest crime," Merriam said at Ribordy's court martial. "It betrays everything soldiers stand for."
Ribordy testified that he had helped stand guard as the prisoners were killed by other members of his patrol in early 2007. He said he approached the scene after the shots were fired and saw three bodies lying in a pool of blood, and then the fourth nearer to the canal.
Ribordy told the court he saw three other members of the patrol - Sgt. John E. Hatley, Sgt. 1st Class Joseph P. Mayo, and Sgt. Michael P. Leahy Jr. - at the scene and smelled gunpowder in the air.
"They all seemed calm," he said.
Ribordy testified that he helped move one of the bodies to the edge of the canal, then push it in.
"I wasn't ordered or asked in any way, shape or form to move the body," he told the court. "I wanted to get it done and get out of there - I didn't want anybody getting in trouble."
He told judge Col. Timothy Grammel that he was now sorry for his actions.
"At the time I believed I did the right thing," he said. "The reason I didn't say anything was because of loyalty to my comrades."
Initially charged with conspiracy to commit murder, which carries a possible life sentence, the charges were reduced Thursday to the lesser accessory to murder as part of Ribordy's plea agreement.
All seven soldiers allegedly involved were assigned to the 1st Battalion, 18th Infantry Regiment, 2nd Brigade of the 1st Infantry Division in Iraq. They are now part of the Germany-based 172nd Infantry Brigade.
Last month, another soldier charged with conspiracy to commit murder, Spc. Belmor Ramos, pleaded guilty and was sentenced to seven months in prison and given a dishonorable discharge. Ramos, 23, testified he had stood guard as the killings were carried out.
Ramos, of Clearfield, Utah, was given the relatively lenient sentence as part of a deal under which he will also testify against others alleged to have been involved in the killings.
Ramos and Ribordy were in the same Humvee during the killings - Ramos manning the machine-gun turret and Ribordy at the wheel, Ribordy testified.
At Ramos' trial and August hearings for Staff Sgt. Jess Cunningham and Sgt. Charles Quigley, witnesses said four Iraqi men were bound, blindfolded, shot in the head and dumped in a Baghdad canal - killings prosecutors said were in retribution for casualties in the unit.
Cunningham and Quigley are awaiting decisions from their Article 32 hearings, the equivalent of a civilian preliminary hearing, to determine whether their cases will go to trial.
In those hearings this year, soldiers who were on the patrol said that the four unidentified Iraqis - likely Sunnis - were taken into custody after a shootout with insurgents and taken to the unit's operating base near Baghdad. Later that night, members of the patrol took the four men out to a remote location and killed them, witnesses said.
Hatley, Mayo, and Leahy have all been charged with premeditated murder, conspiracy to commit premeditated murder and obstruction of justice.
They face a hearing to determine if there is enough evidence for them to be sent before a court-martial, but no dates for the hearing have been set.
Hatley and Leahy were also charged with one count each of premeditated murder and conspiracy to commit premeditated murder in a separate killing near Baghdad in January 2007.
Leahy was also charged with being an accessory after the fact in that incident, a September statement from the Army said, without providing more details.
In other developments: