Authorities on Monday morning imposed vehicle bans in two mostly Christian towns following the attack, and increased security around churches in Baghdad.
Fearing car bombs vehicle bans were imposed in Tilkaif and Hamdaniyah, predominantly Christian towns near the northern city of Mosul.
The coordinated assault came as the Iraqi military predicted that insurgent attacks, though declining, could continue for a few years, raising the prospect of militant violence after the scheduled withdrawal of all U.S. troops by the end of 2011.
Three Christians and one Muslim died in the bombing at around 7 p.m. near a church on Palestine Street in eastern Baghdad, said a police officer who was at the scene. He spoke on condition of anonymity because he was not authorized to speak to the media.
An official at al-Kindi hospital confirmed the death toll and said at least 18 people were injured.
Also Sunday, a bomb exploded near a convoy of American personnel that included U.S. Ambassador Christopher Hill, though no one was injured.
State Department spokeswoman Joanne Moore said the bomb exploded as the convoy was traveling through Dhi Qar province in southern Iraq.
Violence is sharply down in the war that began with the U.S.-led invasion in 2003, but militants still carry out lethal attacks on a regular basis, some seemingly aimed at fomenting sectarian tension. The U.S. military completed a withdrawal of combat forces from Iraqi cities to outlying bases last month as part of a plan to let Iraq take the lead on ensuring its own security.
Gen. Babaker B. Shawkat Zebari, the Iraqi army chief of staff, said insurgents once held sway in cities and provinces, but had been whittled down to a few highly dangerous cells that he expected would continue attacks for "a year or two or three." He said the Iraqi military would get help from American forces if needed, but would also rely on assistance from its own citizens.
"To face terrorism, the Iraqi army does not need tanks or armored vehicles, but needs intelligence, fast communication and people's support," he said "The government has to coordinate with the population to get information about the terrorist cells."
The army chief spoke after meeting Iraq's top Shiite cleric, Grand Ayatollah Ali al-Sistani, in the holy Shiite city of Najaf, south of Baghdad. Al-Sistani enjoys massive support among Iraq's majority Shiites, and the Iraqi military sees the backing of religious leaders as vital to its legitimacy and success.
While violence has diminished since 2007, insurgents exact a steady toll with bombs and targeted killings that would amount to a crisis in most other countries.
In the northern city of Kirkuk, gunmen with silencers in a car waited outside the house of Aziz Rizqo Nisan, head of the provincial audit department, and shot him as he drove to work on Sunday morning. His death was confirmed by local police and the national government's media office in Baghdad.
The motive for the killing of Nisan, a Christian, was unclear. Insurgents commonly target Iraqi government officials and security forces. Ethnic and sectarian tension is high in Kirkuk, a disputed city that Kurds want to annex into their northern region despite Arab opposition.
South of Baghdad, a member of a Sunni militia that is overseen by the Shiite-led government, was found dead with gunshot wounds in his chest in Jurf al-Sakhar town, a police officer said on condition of anonymity because he was not authorized to speak to the media.
The militia, known as the Awakening Councils, includes many former insurgents who joined forces with the Americans and promised to fight al Qaeda in Iraq. The movement was considered a key factor in a drop in violence over the past two years, and has complained about missed payments and crackdowns on its leaders since the Iraqi government took control late last year.
Iraq's Christians have often been attacked by Islamic extremists, and many have fled the country.
Two bombs that were planted in a church in western Baghdad exploded at midnight Saturday, causing some damage but no injuries. Then three bombs exploded near other Baghdad churches at around 4:30 p.m., injuring eight civilians, police said. The fatal bombing followed two and a half hours later.
"The terrorists are determined to hamper the political process in Iraq and not let Iraqis live in peace even after the withdrawal of foreign forces from the cities," said Younadem Kana, a Christian lawmaker. "We demand that the Iraqi government take all necessary measures to protect Christians in Baghdad, and in all of Iraq."
Also Sunday evening, a roadside bomb blew up near a police patrol in Baghdad, killing one civilian and injuring four others, police and hospital officials said.
Half a dozen lawmakers demanded that a general census planned later this year be postponed until after parliamentary elections in January. They argued that the upheaval of war had caused radical change in the ethnic and sectarian makeup of many areas and the results could ignite fresh tension.
Lawmaker Osama al-Nujaifi, a Sunni Arab from the northern city of Mosul, noted that large numbers of Kurds had moved into the oil-rich Kirkuk area amid Arab concerns that they seek to take control. In Baghdad, sectarian violence between Sunni and Shiite Arabs altered the face of neighborhoods as people fled their homes or quit the city altogether.
"The form for the census has an item about the ethnicity of the person, and that would lead to shocking results," al-Nujaifi said at a news conference.