Gears of War 4 beta is coming next month according to Microsoft. The beta will be available for Xbox One on April 25th to everyone but you can play it earlier than that.
Microsoft are allowing players who played Gears of War: Ultimate Edition early access on April 18th through April 24th. You must have play the Ultimate Edition or PC version by April 11th at 11pm Pacific time, which will grant you early access. Lastly, following the open beta from April 25th to May 1st.
There is no current information on what the beta will contain for players.
Gears of War 4 will be available later this year, exclusively for Xbox One.