Why Marvel Stopped Numbering Sequels After Iron Man 3

Iron Man 3 is the final solo film of Robert Downey Jr.'s Tony Stark and the last Marvel sequel that uses the traditional numbering system for a number of reasons.

Iron Man 3 marks a significant moment in the history of Marvel in that it's the final to be a numbered MCU sequel due to a variety of reasons. 

Iron Man 3 chronologically follows the similarly titled Iron Man 2 and Iron Man 3, but the other Phase 1 Marvel Cinematic Universe films 

such as Thor as well as Captain America: The First Avenger could have sequels that have more innovative titles. 

There are a couple of good reasons to believe that Kevin Feige and Marvel Studios altered the formula following Tony Stark's last solo movie, Iron Man 3.

Iron Man 3 represents the remaining pieces from Marvel Studios' Phase 1 

Iron Man 3 is the last Phase that the MCU was still developing its formula after the massive popularity that was The Avengers. 

Phase 1 was not the most united Marvel Phase as Kevin Feige was not in complete control and some films like The Incredible Hulk, were co-produced with other film studios. 

In the context that Iron Man 3 was released in the midst of superhero films developing from the time Sam Raimi's Spider-Man trilogy and the FOX X-Men films

to the current MCU and it is only natural that it was given an unspecific title that is that reflects the naming tradition of the day.