Thus far, the MCU has established an unspoken “trilogies only” rule that has enabled the shared universe to adapt and change, wherein more established heroes like Iron Man and now Captan America graduate out of their own solo trilogies into supporting roles and ensemble films.
Downey’s Tony Stark has already done just that, as he appears in a major role in the third and final Captain America film and has already been announced as part of the cast for next year’s “Spider-Man: Homecoming.”
Putting a finite end point on each hero’s solo franchise allows the filmmakers to craft a standalone arc that develops over the course of three films, applying the same narrative cohesiveness to the trilogies that Christopher Nolan brought to his Batman films.
It is not clear whether there will be a Phase 4 of the MCU or no longer but if there will be none, then “Iron Man 4” seems unlikely.
But if there is going to be a fourth phase to the MCU, it is likely that it would feature the other Marvel superheroes or the sequels to the other films.
From the looks of it, it appears that Marvel Studios has allocated up to three movies for each of its superhero. Iron Man had his three, Captain America just had his three. And the next one who shall have his three is Thor.