https://www.polygon.com/2018/10/18/17995310/xbox-one-x-4k-half-life-2-portal-team-fortress-left-4-dead

Fans of Valve’s classic Portal, Half-Life, Team Fortress and Left 4 Dead franchises may now enjoy these games with 4K enhancements on Xbox One X, Microsoft said today.

Five games from these series have been added to the Xbox One X’s list of enhanced, backward-compatible titles; they join a list of 17 others. The update means they each can now be played at four times the pixel count of their original resolution, a big leap for games that are now more than 10 years old.