Captain Marvel (Brie Larson) will do more than just appear in the still-untitled Avengers 4; she’ll have a significant role in the movie.
A few months ago, Joe Russo debunked the rumors that had the new female superhero joining the cast of Avengers: Infinity War, but made no mention of a possible part in its untitled sequel. Then in September, Larson was reportedly spotted on the set of Avengers 4, but there was no photographic evidence available.
Kevin Feige confirmed during a recent interview with IndieWire that we’ll see Captain Marvel when Avengers 4 is released on May 3, 2019. He added that added that her March 8, 2019 solo movie would be closely linked with the Avengers movie:
We're really focusing on Captain Marvel and the work that Anna (Boden) and Ryan (Fleck) are doing. It's going to be a big part of heading towards this epic conclusion and epic finale of 22 movies over the course of 10 years. That is the focus for the next six movies we have to finish and get out. Really, the focus now is on delivering Captain Marvel, and then bringing Captain Marvel's story into the finale of everything we've started thus far.
The fact that Larson's superhero will show up in the fourth Avengers movie and skip the third does make sense if you take the release schedule into account. It must be also noted that the Boden/Ryan Fleck flick is set in the 1990s, so we need some sort of explanation for Captain Marvel’s absence up until now. Such an explanation probably isn’t going to fit into the plot of an Avengers movie.