The Infinity War trailer released on November 29, 2017, so a similar target area could be in store for Avengers 4, should Disney stick to a similar release schedule.
According to MCU Cosmic, the trailer for Avengers 4 (which may be called Avengers: Annihilation) will release before the end of November.
The site doesn’t claim to know an exact date to expect the trailer, but it does lay down some ideas.
The trailer will likely debut online first (which is often the case these days), or it will premiere on TV during Good Morning America or a football game.