You would expect a 4k to be able to show full 4k when youtube goes up to 8k

Notice in both the snapshots below that the video resolution is being played at only 1080p or 1440p, and not the full 2160p that would be possible on a computer as shown above.

(UPDATE - HDR FIXED on our Samsung now plays Full 4K Youtube)

Snapshot of LG 4k UHDTV

Resolution is 1080p although we did get 1440p also, but notice the 720 *3 (screen) Dimensions. I bet LG never thought we would see this as it gives away how the picture is made, so maybe it's one good thing for the buyer.
Today LG were made aware of this and filled a VOC to be passed on to managers who may then contact youtube.

Snapshot of Samsung 4k UHDTV

Resolution is 1440p, also notice the 1080 *2 (screen) Dimensions.

Samsung have been made aware on the 9th and looking into it. Their support managed to let me have an update ahead of public release.

My video of the Youtube app in action on a UHD Television

One way that did work was by using the built-in browser, going to youtube and choose 4k just like on a computer.

Although you might find it lagging and buffering at first, it does smooth out.

Make sure you have the latest updates for your TV, which can be got from the manufacturers website under support or where you have registered your device.

So how do you watch 4K YouTube videos online?

YouTube has supported 4K Ultra HD video since 2013.

YouTube Ultra HD videos use a codec called VP9.

The VP9 codec is royalty-free, meaning its adoption rate could potentially be higher, and in 2015 saw 8k videos being supported.

Google also implemented the VP9 codec into its Chrome browser and YouTube back in 2013, enabling both being able to support 4K streams for some time. If you search 4K content within YouTube right now, you'll be able to select 4K as a quality option on each video on a computer, bt on a Televison this is missing and instead is in some kind of auto mode.

The problem could be with HEVC/H.265, VP9 that needs compatible hardware - i.e. a 4K screen - to watch.  Most PCs nowadays should support the VP9 codec, but with televisions it seems this proves if the TV is True 4k or some hybrid.

Whathifi say "If you don't have a compatible display, the video will be downsampled to the maximum output of your monitor."

Where else can you stream 4K video?

There are a few services that offer 4k:

Netflix was one of the first video-on-demand services to announce it would be supporting 4K streaming, which went live in the UK.

Amazon also offers 4K content through its Prime Video streaming service.

You can watch 4K video via the Amazon Prime Video app on compatible TVs and the latest 4K Fire TV box. You can see a full list of Amazon Prime Video devices here - note this is a list of compatible devices with the movie streaming app and not necessarily 4K content.

UltraFlix is a another 4K streaming network from NanoTech, which has apps available on Samsung, Sony, Vizio, Sharp and Hisense TVs.

Sony has launched its own 4K Video On Demand service in the form of Video Unlimited 4K, but is only available in the US.

What about 4K TV channels

BT became the first broadcaster to launch an Ultra HD channel to the UK, with BT Sport Ultra HD in August 2015. 

Sky is the "UK's most comprehensive Ultra High Definition service" through its Sky Q platform launched it's ULRA HD service in August 2016.

For many, getting a fast broadband connection to support 4K streaming (realistically at the very least 30 Mbps) isn't fully possible right now.

Therefore a better way to get 4K content into the home would be on a Blu-ray disc format.

Ultra HD Blu-ray that can handle resolutions up to 3840 x 2160 are available but slow to take off.