Mike Ybarra says all games on Xbox One S will be upscaled to 4K.

The slimmer version of Xbox One was revealed at E3 with an amazing reception by the fans. However, many fans were confused if the console upscale all the games to 4k if supported.
Mike Ybarra, Xbox & Windows Gaming Platform at Microsoft has address the confusion saying “Yes” – it does upscale the games to 4k.

Xbox One S has an additional CPU and GPU power over Xbox One. According to Coalition Studio head, Rod Fergusson, Xbox One S deliver a more consistent technical performance. He further said that Xbox One S has raw CPU and GPU power, engineers at Coalition were able to take advantage of it, by reducing the frequent frame rate drops in the demanding section of the game.
See the reveal of the console below.