The Samsung S10 camera is Capable of taking 4K video, but S10 display isn't 4k compatible so it just plays in 1440p in YouTube app. Yes the display needs to support 4K to play video in 4k resolution.
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What resolution happens when the 4k video is cast to a 4k TV? Does it still only do the resolution of the phone? Or what if the phone is running Dex on the 4k TV. is Dex playback limited to phone resolution?