Lieutenant-Colonel Kifah Muhammad, a local police officer, said that two women, a man and a child were killed on Thursday.
The city, north of Baghdad, has seen a sharp increase in violence over the past week.
Fifteen Iraqis died there on Wednesday when a bomber rammed his bomb-laden car into a US patrol and troops retaliated by opening fire.
Clashes between US-led troops and Iraqi fighters later erupted in several parts of Baiji, prompting troops to seal off the oil refinery in the north of the city to protect it from attack.
In Kirkuk, an ethnically mixed oil-producing town 100km north-east of Baiji, a roadside bomb that appeared to target a US military convoy killed two Iraqi civilians on Thursday.
Police in Kirkuk said the blast damaged several vehicles.
US military says it has achieved
The same day, Iraqi fighters attacked the provincial governor’s office in Iraq’s third largest city, Mosul, killing one of the governor’s bodyguards and wounding four more, the US military said.
The fighters fired 10 mortar rounds at the building in the
northern city, 390km from Baghdad, setting ablaze a fuel tanker parked nearby.
Governor Duraid Kashmula, whose predecessor was
assassinated in July, was unhurt.
The fighters also fired six mortar rounds at a US military base
in Mosul, but there were no injuries, a US military
Violence has flared in the north in the 10 days since
US-led forces attacked Falluja, where fighters had established control.
US troops say they now control Falluja after killing
hundreds of Iraqis, although fighting has continued in parts of the city.