Robert Downey Jr. is set to return as Iron Man in the upcoming 'Avengers: Age of Ultron' by Joss Whedon, but he remains uncommitted in reprising that role in a potential 'Iron Man 4'. Last year, in an interview with Variety, Downey indicated that there were no plans for a fourth installment of the series. "There isn't one in the pipe. No, there's no plan for a fourth 'Iron Man'", he said.
While Marvel Studios is undoubtedly keen on producing an 'Iron Man 4', with the previous installment grossing over $1.2 billion in the box office, they have yet to indicate plans for such a production. Instead, they have announced projects featuring other lesser known characters, such as Black Panther, Ant Man, and Doctor Strange. Marvel appears to be hedging their bets as they attempt to launch new franchises as Downey has long been rumored to be disinterested in an 'Iron Man 4'.
Another potential stumbling block to an 'Iron Man 4' is the salary negotiation with Downey. According to various reports, Downey took home an estimated $40-50 million for 'Iron Man 3'. Such a sum was well worth the cost given the massive success of the movie, but it remains a tricky situation for the studio as Downey has all the leverage in the world to negotiate a much higher salary.
Of course, Marvel is free to find another actor for the role, but that has wider implications considering the impact it will have in the continuity of the Marvel Universe. The Robert Downey version of Iron Man plays a critical role in the Avengers franchise and will also feature heavily in 'Captain America 3'. The storyline for the third Captain America movie is based on the comic book story 'Civil War', wherein Captain America and Iron Man will find themselves in opposite sides of a moral and political battle. Losing Downey would be a blow for potential followups to that storyline.
As it stands, Downey is slated to appear as Iron Man in 'Avengers 2', 'Captain America 3', and 'Avengers 3'. Beyond that, it appears that Downey's Marvel future is steadily coming to a close.