This article says that RDJ doesn't actually claim to be in the film. As much as I like his Iron Man, if Marvel wants a sustainable film series they will need to replace actors (similar to the Bond films) eventually. Why not sooner rather than later.
I think it would be great to do it now. RDJ really wrapped up "his" Iron Man in 3.
Also, if new actors were gradually introduced with the established ones over time it could actually serve to create a smoother transition. Hell, its comic book characters--it would be like introducing new artists or new writing teams into ongoing series.