The shooting occurred in Gbadolite, in the north-west of the country, where supporters of the presidential challenger, Jean-Pierre Bemba, clashed with guards of Nzanga Mobutu, a supporter of the incumbent President Joseph Kabila.
Among those killed was at least one of Bemba's troops, according to the local headquarters of the UN mission in the country (MONUC) in Mbandaka, the capital of the western Equateur province where Gbadolite is located.
Three policemen were also killed and one of Mobutu's guards injured, MONUC added.
Mobutu's brother, Philippe Albert Mobutu, meanwhile told AFP that a fifth person, a civilian, was also killed while fleeing the violence in Gbadolite. The fifth death was not yet confirmed.
Gunfire erupted when troops loyal to Bemba surrounded the headquarters of a radio station, while Mobutu, son of the country's former dictator Mobutu Sese Seko, was inside, a MONUC spokesman said.
It was not known why Mobutu had been visiting the radio station, Radio Liberty, which is owned by Bemba. But a spokesman for Bemba's camp, Fidele Babala, alleged in a statement to AFP that Mobutu's armed forces had occupied the radio headquarters and fired at journalists.
The radio station remained surrounded by Bemba's troops Thursday evening, with Mobutu still inside, local sources said. Philippe Albert Mobutu said negotiations were under way to release his brother from the blockade.
Tension is high in the vast, war-scarred central African nation which is due to vote on Sunday in a run-off poll between Bemba and Kabila, in the second round of the country's first multiparty elections in 46 years.
Violence broke out elsewhere in the country on Thursday.
A supporter of Bemba was stabbed to death in Lodja, central DRC, prompting locals to set fire to houses and two radio stations.
In the capital Kinshasa at least three detainees were shot dead when police crushed a prison riot. The rioters were demanding food after the prison, where detainees rely largely on food brought in by relatives, was closed to visitors following a reported break-out of people sentenced to death for a political assassination.
Stones were also thrown at supporters of Bemba as they held a rally in the eastern town of Goma.
"People are overexcited," one western military source told AFP on condition of anonymity, adding that the next few days would be delicate.
In Brussels, European Union foreign policy chief Javier Solana called for voters in DRC to turn out in force to vote on Sunday "in peace and dignity".
"The stakes are of historic importance," the EU official said.
Bemba meanwhile told journalists in Kinshasa that he would accept defeat by Kabila, the favourite to win, if the polls were "free, democratic and transparent".
He raised the spectre of further violence when his supporters stage a mass rally in the capital on Friday.
"I do not want blood to be spilt during this demonstration," he said.

Copyright AFP (Agence France-Presse), 2006