The Gears of War 4 beta will kick off on April 18th, 2016 and if you have played Gears of War: Ultimate Edition on Xbox One or Windows 10 tokens will begin to roll out in waves for early beta access. Early access for the beta ends on April 24 and at that point all qualifying Xbox Live Gold members on Xbox One will then be invited to join the beta which ends on May 1st, 2016.
Additionally, the Gears of War forum announced an extended timeframe for Gears of War: Ultimate Edition players to qualify for early access to the Gears 4 beta from March 1st to April 11. Just log into Xbox Live to play Gears of War: Ultimate Edition on Xbox One or Windows 10 before 11PM PDT on April 11 to qualify.
Source: Gears of War