That question is currently up in the air, but Robert Downey Jr. seemed to insinuate in a recent interview that it's up to Disney and Marvel.
“It’s down to Kevin [Feige, Marvel Studios president] and Ike [Perlmutter, CEO of Marvel Entertainment] and Disney to come to us with what the proposal is, and that’s on us to agree or disagree,” Downey told EW.com. “When things are going great, there’s a lot of agreement."
Regarding things going great, Robert Downey Jr. answered the questions on the set of The Avengers 2, where he indicated everything is presently a-okay.
“Right now, this has just been swell, hasn’t it?” he said with a big smile. “This has been a really good one and it feels good and we’re having a good time.”
Downey Jr. is presently the highest paid actor in Hollywood, earning $75 million this past year, thanks in part to last year's highest grossing movie, Iron Man 3.
It's previously been said that when a new Tony Stark is cast, they could go the route of a James Bond, meaning sticking with the continuity, but using a new actor (and one at a much cheaper cost).
It's possible with Marvel Studios recently announcing a slate of new movies that we could find out if RDJ will return for Iron Man 4 this weekend at Comic-Con.