It's learned that Marvel Studios and Disney have retitled the two Avengers: Infinity War movies.
The first movie (Avengers 3) will now be simply titled "Avengers: Infinity War" and has the same release date of May 4, 2018.
Marvel Studios has dropped "The Infinity War - Part 2" title from the fourth Avengers film, with the film currently having no title other than "Avengers Untitled" (meaning yet to be named). Avengers 4 (or whatever they call it) is still planned for a May 3, 2019 release.
The change has to to due with the fact that both movies are different from each other and are not one big movie cut in half (à la The Hobbit).
Assuming nothing else has changed, the Russo Bros. are directing both Avengers movies and are due to begin filming this November starring Josh Brolin as the Mad Titan, Thanos.