String of attacks
Three of the officers died when a homemade bomb targetting a police patrol went off, while the fourth died in a drive-by shooting at a security checkpoint.
An Iraqi soldier was also killed and 13 people wounded, including four other troops, by a car bomb near the city of Mosul, a police official said.
The string of attacks came as funerals were being held for some of the 72 victims killed in the bomb blast at a crowded market in Baghdad's Sadr City a day earlier.
Relatives of two police officers killed in the attack marched through the streets of the city, behind vehicles carrying the two coffins draped in Iraqi flags.
Hundreds of angry Iraqis gathered around the wreckage of the market bombing, demanding better protection from the government when US soldiers pull back to rural bases.
Nuri al-Maliki, the Iraqi prime minister, insisted the country's security forces were capable of protecting citizens after US forces pull out.
"We assure you of Iraqi forces' readiness for the mission, despite some security violations, and we assure you that we are now more stable and steady," he said.
'No perfect situation'
Raed Jarrar, a Washington-based political analyst, told Al Jazeera: "The US withdrawal is a necessity and Iraq is more than ready to end the occupation, but of course that doesn't mean that Iraq is in a perfect situation yet.
"There is a lot of work still to be done to enhance the Iraqi security forces, to enhance the Iraqi political process and draw in all Iraqis around the country."
The wave of violence comes just days after the US military formally handed control of Baghdad's Sadr City area - the scene of Wednesday's bombing - to local forces.
US forces are also due to withdraw from all cities and major towns of Iraq by the end of this month, including Mosul and Kirkuk, where violence levels remain persistently high.
A "small number" of US troops would be left in some Iraqi cities after the June 30 deadline at so-called Joint Security Stations to train and advise local security forces, a military spokesman said.
The US military will also continue to provide intelligence and air support.