Here you can see how uncharted would look at 4k(upscaled) it's off screen but I can see the difference. Obviously would look better on the pro but it's an idea. go to 4:35 for a better view

also one question, is the ps4 slim going to upscale games too?

I think all 4k TV's upscale sub 2160P content up to 4k anyway, so with the Xbox One S, it is just done internally within the console before being sent out to the TV. The only real benefit of this is less input lag as it is being upscaled prior to being output. So I'm assuming the PS4 Slim on a 4K TV will 'look' the same but maybe features a tiny tiny bit more input lag?? Maybe someone can clarify.

The HDR issue will be seen when Sony put out the HDR patch covering all PS4s, which 99.9999999% of people thought was impossible to do because of the HDMI being rated at 1.4...

Just out of curiosity, how did someone do this though XB1S?

I don't have a xbox but I think you have to connect the hdmi cable of your ps4 or wii u to the xbox one and then the xbox one to the tv/monitor, and the xbox one has an application to do this. something like that.

I believe he connected his PS4 to his Xbox One S via the HDMI input port in the back of the Xbox One S.

 Oh, I thought MS removed the HDMI in port from the S. I couldnt find it on mine, but then again I didnt look too hard lol

Hehe oh it is definitely there. What I find very strange is that MS kept the HDMI in port(rarely used) and optical port in the Xbox One Sand added a UHD drive, whilst Sony stripped the optical port out of the Slim and did not include UHD blu ray support into the Pro, even though they are winning the generation by far! I would have expected MS to strip back the functions in this case in order to lower the cost of the console.

I recall reading an article on Forbes praising the XB1S scaler. I believe the writer claimed it is superior to the scalers employed by most of the cheap 4k displays, so hooking the PS4 up to an XB1S might not be such a terrible idea.

Uncharted 4 won't have HDR compatibility until the pro patch comes out, (at least I don't think so, as the ps4 won't support HDR until 4.0)so it's not in HDR yet. Plus, the HDMI in feature isn't wonderful, in my experience, as it tends to be laggy and a little fuzzier. So if it looks good in those conditions, that's a good sign for the pro :)

What do you mean, rarely used? Why would HDMI-in be rarely used?  I'd be surprised if this was a rarely used feature, especially on the Xbox One S that has an integrated IR blaster.