I have a theory...
Remember the scene from the trailers where they were all charging in Wakanda and Hulk was there? Wasn't in the movie. Also, there was a toy and a lot of talk about Hulk busting out of the Hulkbuster armor, but that never happened.
What if those were scenes from Avengers 4 that they shot while filming Avengers 3? What if we get the Battle of Wakanda fought all over again because of some fancy time travel stuff and they filmed that at the same time they filmed the scenes for Infinity War?