The marines have now lost 14 men in the past nine days on various operations around al-Anbar - home to Falluja, the town which has witnessed some of the most protracted and violent clashes.
Speaking of the latest incident, the US military said in a statement: "Four marines assigned to First Marine Expeditionary Force were killed in action July 6 in the al-Anbar province while conducting security and stability operations."
On Monday, two marines were killed in action and one died later from wounds received during an operation in the region.
A resistance group later claimed responsibility for the attack.
"With God's help, about 100 of the Umar battle lions laid an ambush Monday for the soldiers of Satan, the Americans, in al-Saqlawiya region, west of Baghdad," the Tawhid al-Jihad said in a statement published on an Islamic website.
"Thanks be to God, the toll was two helicopters down, including an MH-47 Chinook, and two Humvees destroyed with their occupants on board," it said.
It was impossible to verify the authenticity of the claim.
The latest deaths brought to at least 644 the number of US soldiers killed in action in Iraq since last year's US-led invasion.