It is very unlikely that actor Robert Downey Jr. may be stepping in to play Iron Man if "Iron Man 4" does release by 2019 or 2020. However, fans speculate that the actor may still come back as Tony Stark and serve as a mentor to the new person who will don the Iron Man suit.
The report points to an interview with Empire Magazine where Downey Jr. admitted he is getting old for the action-drive role. At some point soon, Downey Jr. will need to relinquish the role and allow Marvel Studios to re-cast Tony Stark or do what the comic books do: have someone else other than Tony Stark step in to be Iron Man.
"I also recognize that I'll be turning 50 by the time I promote this movie. The clock is ticking down on the amount of memories and participation that I would allow myself and not embarrass the medium with," explained the actor at the time.
The report also points out that by the time "Iron Man 4" begins production, Downey Jr.'s contract would have expired and it may be expensive for the studios to sign him back on, given how much he has been paid for his participation in the two "Avengers" movies and "Iron Man 3."
One of the most popular theories spreading about the upcoming film is Ty Simpkins, who played Harley Keener in "Iron Man 3," may be old enough by 2019 to star as the new Iron Man. It is possible that Tony Stark, realizing he is getting old, appoints Harley as his successor.
Fans also speculate that it is possible for Rhodes (Don Cheadle) to become Stark's successor. However, it is also speculated that Rhodes will die in this year's "Captain America: Civil War," and if this proves true, then it would be impossible for Rhodes to be the new Iron Man.