The four are Martin Fayulu, the runner-up in the 2018 vote; the influential former governor of Katanga province, Moise Katumbi; former prime minister Augustin Matata; and prominent MP Delly Sesanga.
"We have decided to bring our ideas and strengths together," the four said, "to lead joint action to ensure transparent, impartial, inclusive and peaceful elections within the constitutional timeframe."
They said their "first action" would be a march in the capital Kinshasa on May 13 "to denounce worsening security situation, the high cost of living and the poverty of the people (and) the chaotic electoral process."
The one-round presidential poll is scheduled to take place on December 20, at the same time as parliamentary, provincial and local elections.
Friday's announcement by the four was made in Katumbi's stronghold, the southeastern city of Lubumbashi.
The joint declaration has stirred speculation that the four may eventually coalesce around a single candidate to challenge incumbent president Felix Tshisekedi, who has already said he will seek re-election.
Nearly a dozen people have already declared their intention to stand.
The one-round presidential poll is scheduled to take place on December 20, at the same time as parliamentary, provincial and local elections.
Democratic Republic of Congo has a long history of political turbulence. The December 2018 vote saw the nation's first peaceful handover of power after authoritarian ruler Joseph Kabila stepped down voluntarily after 18 years in office.
The victor in 2018 was long-term opposition figure Felix Tshisekedi, but his win came at the cost of furious accusations that the results were rigged.