We really don't know anything about Avengers 4, but fans have assumed that one major new character will be in it, and that character is Captain Marvel. We've seen some proof of her involvement, but now, for the first time, Marvel Studios' Kevin Feige has confirmed that the events of the Captain Marvel solo film will tie into Avengers 4 and continue that story arc.
During an interview with IndieWire, Kevin Feige was asked about the future of female characters in the Marvel Cinematic Universe particularly in regards to Captain Marvel (Brie Larson) and this is what he said:
"We’re really focusing on ‘Captain Marvel’ and the work that Anna and Ryan 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 focus for the next six movies we have to finish and get out."
Feige then went on to say:
“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."
I'm excited about seeing Captain Marvel brought to life on the big screen and I can't wait to see how the movie turns out! As of right now, all we really know about the solo film is that it's going to be set in the early 90s, and feature Carol Danvers and Nick Fury somehow getting caught in the middle of the Skrull and Kree war.
Things are going to get even more interesting when we see how she ends up teaming up with the Avengers in Avengers 4! As of right now, all we can really do is speculate on how all that might play out.
Captain Marvel will be released on March 8th, 2019, while Avengers 4 is set to be released on May 3rd, 2019.