The Xbox One X have a native support for 1440p(2560 x 1440) monitors confirmed by Microsoft official.It has been hugely promoted by Microsoft as 4K product.
4K resolution is very popular among the gamers community for high end gaming experience, who moreover focused on frame rate.The X One X will support variable framerate on FreeSync monitors, although no official confirmation has been given on it yet.
This is bit different with Sony PS4 approach, however many PS4 games are being rendered at 1440p when upscaled to 4K TV. The PS4 pro have support for both 1080p and 4K resolution, but 4K output will only work with 4K screens. The major issue with PS4 pro you will only get output of 1080p resolution when playing 4K game at 1440p monitor.