The most recent fan theory suggests that this could be because Avengers 4 picks up four years after Thanos's snap changed the world as the superheroes knew it. The film also has to deal with how the Avengers rise above differences between them.
Characters are encouraged to “fight the good fight.” It also explains the different look of the film and the state-of-the-art suit that Hulk is sporting. Bruce's truce and Captain Marvel’s homecoming will all make sense if this theory is true.