"The charge had been placed on the side of the road when it exploded and killed all four policemen," the source said.
He added that police had taken the bodies of the victims to the Forensic Medicine Agency.
Elsewhere in the province of Anbar, three ISIL members were killed in clashes with Iraqi police and army troops in Fallujah on Monday, according to tribal sources.
The spokesman for the organizing committee of an anti-government protest camp in Fallujah, Mohamed al-Bagari, said the city was the site of several battles between armed tribesmen and the Iraqi army on the eastern international highway.
"The army bombed a number of homes with artillery, leaving five civilians injured," al-Bagari said. "A large number of homes were destroyed as well."
The province has been rocked by violent clashes that broke out after Iraqi security forces dismantled a months-old anti-government sit-in outside the provincial capital Ramadi in late December.
As of Thursday, the death toll from the clashes had reached a least 57, according to medical and tribal sources. Around 300 were also said to have been injured.
The sit-in dispersal came two days after Sunni lawmaker Ahmed al-Alwani, a prominent opponent of Prime Minister Nouri al-Maliki, was arrested by Iraqi forces during a raid on his Ramadi home. Six people were killed in the raid, including al-Alwani's brother.