Downey Jr. has played the character in a trilogy of "Iron Man" movies, as well as the first superhero ensemble "Avengers" film, which made $2.7 billion worldwide.
The next film will start production in March and will be led again by writer-director Joss Whedon. A theatrical release is expected in May 2015.
The 48-year-old actor had previously suggested "Iron Man 3" might be his last spin in a stand-alone "Iron Man" film. Marvel's announcement made no mention of an "Iron Man 4" release.
The Associated Press contributed to this report.