The attack happened at the Yarmouk traffic intersection in Iraq's third-largest city, 225 miles northwest of Baghdad.
The bodies of the four victims, including one whose head was almost severed, were seen lying on the road alongside their burning car.
Police Capt. Zeid Waseem said police received reports that three foreigners and their Iraqi driver had been killed. The nationalities were not immediately known.
In other developments:
Another person was killed Thursday when seven mortar rounds struck near local government offices in Mosul, police Capt. Mohammed Abbas said.
The killings were the latest violence in the city, which was initially peaceful after the U.S.-led invasion but become a worrisome trouble spot since U.S. and Iraqi troops invaded the insurgent stronghold of Fallujah, west of Baghdad, in November.
Meanwhile, U.S. military spokesman Lt. Col. Paul Hastings said security forces on Thursday found the bodies of six unidentified Iraqi civilians in western Mosul who had been shot dead.
Residents found another two Iraqis wearing civilian clothes who had been shot in the head in Hamdaniyah, about 20 miles east of Mosul, police said.
Since Nov. 10, about 160 bodies, including many affiliated with the Iraqi National Guard and police, have been found.
Soon after the U.S. assault on Fallujah began, insurgents launched an uprising in Mosul, overpowering police, looting or burning police stations — and prompting an offensive by U.S.-Iraqi forces to put them down. Since then, police say insurgents have been targeting security forces and police there in a bid to strengthen their grip.
With six weeks of campaigning left ahead of the crucial vote for a 275-member assembly, interim President Ghazi al-Yawer predicted regional and international interests will spend millions of dollars to influence the balloting — a statement aimed primarily at Iran and Syria.
"There are many parties, regional and international, who want to serve their own interests and they want to have friends in power in Iraq," al-Yawer said. "We think that millions of dollars will be spent on the elections process from outside the country. We hope that this will not happen and that the money and the decisions will be Iraqi."
Al-Yawer's comments came a day after Defense Minister Hazem Shaalan accused Iran and Syria of supporting terrorism in Iraq.
Interim Prime Minister Ayad Allawi and U.S. Gen. George Casey, the chief of the multinational force in Iraq, separately accused Syria on Thursday of hosting former Saddam regime members involved in backing Iraq's insurgency.
Syrian Foreign Ministry angrily rejected the accusations as "baseless."
"Syria sees the repeat of fabricated accusations as reflecting the wish of some people to hide the real reasons behind the deterioration of the situation in Iraq, and to mislead the public opinion," the ministry said in a statement.