I am operating under the assumption that RDJ is talking about there maybe being an Iron Man 4. And I am assuming that Phase 3 ends with Avengers 3 which is everyone versus Thanos. Again, this is all assumptions, and thinking they wont drag out the battle versus Thanos into Phase 4, and that currently RDJ's last contracted film is Avengers 3.
I am also assuming that if RDJ is going to finish off his contract as Iron Man, he needs to go out in a blaze of glory. He needs to die a hero, and what better way to do it than to finally be the one that lays down on the wire for someone else, meaning he dies saving Cap's life. Then combine that noble sacrifice with finally seeing Cap wield Mjolnir. And when I see Cap wield it, it better be in the time of ultimate need. It better be a moment to make the Hulk stuff in Avengers seem like kiddie stuff. And what moment could be more dire than in the final battle against Thanos?
Again, all my wishes are just assumptions that Avengers 3 is the final appearance of both RDJ as Tony and Evans as Cap.