Gears of War 4 uses the Unreal Engine 4 to power the game and is a major milestone for the engine. Considering Epic Games is the maker of Unreal Engine and Gears of War was originally a IP of theirs until they sold it to Microsoft, it is fitting that Gears of War 4 does the engine proud with how technically impressive it is across Xbox One and Windows 10.
Digital Foundry goes into why this titles does a great job using the UE4 with the above video.