UPDATE: Gears of War 4 director Rod Fergusson clarified on Twitter that for a limited time if you buy Gears of War 4 you'll receive all the previous Gears games for Xbox 360.
This includes pre-orders and buying the game shortly after launch. All games will be backwards compatible on Xbox One.
ORIGINAL STORY: Gears of War 4 will have both Cross-Buy and Cross-Play support with Windows 10, Microsoft announced at its E3 press conference moments ago.
Buy it on one platform and you automatically get it on the other. All modes will be compatible with each other in co-op, including the new Hoard mode. Achievements will carry over as well.
We then saw the following video of the game's campaign. Note the new gun that shoots saw blades, Kait's non-chainsaw bayonet, and a grizzled Marcus Fenix looking a couple of decades older.
Gears of War 4 is due on 11th October.
Read up on what else Microsoft announced at their conference with our E3 schedule guide.