Marvel Studios has yet to announce any plans for "Iron Man 4" and Robert Downey Jr. has yet to extend his contract for more films. Due to this, along with Downey Jr.'s age and high fee, it is rumored Tony Stark will be replaced by any of two likely candidates when "Iron Man 4" does become a reality.
According to Ecumenical News, there is a chance of Tony Stark passing the legacy of Iron Man to a young female named Riri Williams. This is based on the new "Invincible Iron Man" comic books where Tony Stark retires and Riri is chosen as his successor.
Riri is a teenage African-American girl who is so smart she made her own mock Iron Man suit using scraps. She is then discovered and Tony names her as his official successor, giving her the name Ironheart.
Game N Guide, on the other hand, points to an older rumor suggesting that Harley Keener, played by Ty Simpkins, may be the successor to Iron Man. Harley was introduced in "Iron Man 3" and like Riri he had proven to be a genius inventor even at a young age, helping Tony fight his enemies even without the help of the Iron Man suit.
Many fans speculate "Iron Man 4" may play out like the first "Ant-Man" movie, which saw Hank Pym (Michael Douglas) give his Ant-Man suit to Scott Lang (Paul Rudd). Pym was still present in the film but as an advisor to Scott and it is rumored Tony will be an advisor to Riri or Harley in "Iron Man 4."
If this proves true then Robert Downey Jr. may be paid less given he will no longer be the main draw for the film and his contract with Marvel may be altered to only include him in a small role or in a cameo.
Marvel Studios have not confirmed or denied any of the rumors and no announcements have yet been made for "Iron Man 4."