Along with the release date change for Avengers 4, we also received the official Avengers 4 title, which is Avengers: Endgame.
As for the reasoning behind the Avengers: Endgame release date change, it all has to do with spoilers.
Marvel Studios has confirmed that Avengers: Endgame will release one week earlier, on April 26th, 2019.
It was teased that this would be a huge week for Marvel Cinematic Universe fans, and it already has with the release of the second Captain Marvel trailer and Avengers: Endgame.