Depends on the game and the quality of the TV. Games like Batman: Arkham Knight, Battlefront etc all look better on the PS4 as the game runs at higher native resolution. The XB1s doesn't increase the resolution but just upscales (stretches to fit) the 4K screen size before outputting to the TV. The PS4 will output at 1080p and then the TV will upscale that to 4k to fit the screen so the PS4 still has the edge over games which are running at a higher native resolution to start with. The on...
If you still have an XB1, you can see if your TV upscales as well as the XB1s. Both of these consoles run games at exactly the same resolution, level of detail, levels of visual effects etc. Therefore if you plug both into your 4k TV, the only difference should be where the upscaling takes place - with the XB1s, its by the console where as with the XB1 its by the TV. I have tried this with my TV and I can't see any noticeable difference so my TV must upscale as well as the XB1s.