Just because Robert Downey Jr is open to continuing his role as Tony Stark in the Marvel Cinematic Universe doesn't mean Iron Man 4 is on the schedule. Somehow a rumor got started that Iron Man 4 was happening, but when asked about it at the Toronto International Film Festival premiere of his new film The Judge Downey shut that all down fast.
“There isn’t one in the pipe...No, there’s no plan for a fourth ‘Iron Man.'”
That's pretty cut and dried, although I'm sure it'll get twisted by shitty outlets looking to write misleading headlines.
It's important to note this doesn't mean Iron Man 4 is impossible - it could very well happen at some point in the future. But Marvel seems to be happy to give the series a rest while it builds the rest of its universe, and that's a smart idea - Guardians of the Galaxy has the first Iron Man beat at the global box office, and it looks like it could sneak past Iron Man 2 at the domestic level (and possibly the global as well - there are more major markets in its future).
If Guardians had fizzled or underperformed Marvel would probably be pitching Iron Man 4 ideas right now, but the fact that the movie is a huge hit shows they don't need their core franchise. Because the reality is that Marvel is the franchise.