The thing is, though, by and large in most big franchise movies, the lead actor is ultimately replacable. Be it Sean Connery as James Bond, Toby Maguire as Spider-man, Michael Keaton as Batman, William Shatner as Captain Kirk or Ed Norton as the Hulk, the character is generally bigger than the actor.
There's no doubt that RDJ brought a hell of a lot to Tony Stark and Iron Man and he will be a hard act to follow. But equally, I can see Marvel thinking 'Well, he's getting on a bit, we'll have to replace him eventually anyway, why pay him another $50m for Avengers, which is bound to be a dead cert hit anyway?'
Indeed, if Marvel did want to replace RDJ (or at least balked at his salary), an Avengers film would seem to be the perfect time to do it, rather than Iron Man 4. No better time to introduce a new actor than among the ensemble, rather than have him carry the whole IM franchise on his shoulders. Even if people don't particularly warm to this new Tony Stark, they'll still have the other characters to watch. And, of course, if he or she proves a success, then they're all set to star in IM4.