Did you forget what's happened on Sera? Don't worry as Gears of War 4 has you covered. The beginning of the game takes you through three key points in the history of the series.
There's securing the Hammer of Dawn, Emergence Day, and the Locust's seige of Anvil Gate. None of these have been in the game before so it'll be fun to experience some key events that were referenced to in past games.
The third game had you playing through a prologue and the fourth one will as well as you get to experience these events through eyes of a new character. October 11th, just a scant two weeks away, will be when you go do this all for yourself.