DR Congo has been hit by years of instability, with rival militias fighting for control of territory.
The conflict has been worsened by the failure to hold elections last year.
"It's a mega-crisis. The scale of people fleeing violence is off the charts, outpacing Syria, Yemen and Iraq," the Norwegian Refugee Council's DR Congo director, Ulrika Blom, said.
In a new report, the Internal Displacement Monitoring Centre said that an average of 5,500 people fled their homes every day this year.
The reasons include new armed conflicts, a rise in existing conflicts and the delay in holding elections, the report said.
Despite there being four million displaced people, as well as more than seven million struggling to feed themselves, international aid has been slow to materialise, Ms Blom said.
"If we fail to step up now, mass hunger will spread and people will die. We are in a race against time," she warned.
DR Congo's Minister of Information, Lambert Mende, has however disputed the aid agencies' report, saying that the number of the displaced people is "less than one million".
He told BBC's Focus on Africa programme that the displaced people were, contrary to the report, returning home from neighbouring countries.
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