The full title of Avengers 4 might be Annihilation, according to a rumor posted by MCU Cosmic,

The writer says this information was hidden in the Russo brothers’ recent “look hard” photo.

If this is truly the title, Avengers 4 will share its name with a 2006 Marvel miniseries. The phrase may refer to Thanos wiping out half the population at the end of Infinity War, and also indicated that the movie may go in a dark direction.