Some people, like myself may have experienced issues with 4K content on their 4K TVs on the YouTube app. I have the KU6400 and the recent update brought HDR to the YouTube app. Though 4K content is available on YouTube it only seems to play in 1080p. Apparently this has to do with Internet speeds. You have to be receiving 25MB speeds to be able to watch 4K on YouTube.
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I think the TVs WiFi reception is poor because nothing I did worked when trying to use it on WiFi so I ran a cat6 Ethernet cable directly from my router to the TV and it fixed the problem I now get full Wi-fi speed at the TV. Cable is very cheap if you can go that route. Hope this has helped.
Im have the same problem in fact still waiting for a email reply from Samsung, youtube 4k videos only work occasionally and my wifi is more than 50mbs download speed but much less than 20 at the tv so I have pluged in a TP-Link extender only rated at 300mps but I immediately received 4k.
I believe that the wifi receiver built in to the TV is of poor quality as it cant seem to receive the correct amount of internet.