Gears of War 4 has been out for awhile now, and so the safe assumption is that if you haven't bought it by now, you're probably not going to—not without some incentive, anyway. Such as, perhaps, the opportunity to play it for up to ten hours during the free trial period that begins next week.
From June 9 to June 15—or possibly longer, a point of confusion we'll get into below—a "limited time trial version" of Gears 4, with the first act of the single-player campaign plus full access to the Versus and Horde modes, will be available, offering up to ten hours of cumulative play time. Progression earned during the trial will carry over if you purchase the full game, and special bonuses for new players including "guaranteed characters, weapon skins and new Horde Skills" will be granted as well.
Update: The announcement of the trial implies that while the free play period will run for the full week, the demo actually has to be downloaded over the June 9-11 weekend. However, a Microsoft rep confirmed that the trial version of Gears of War 4 will be downloadable during the entire trial period.