I just got my new 4K Apple TV, iTunes movies look absolutely stunning , 4K HDR @60Hz on my Sony TV looks awesome.
On the side note, finding out that my Epson 6040 UB can't handle HDR @60hz, completely sucks..... , it does 4k @30 4:2:2 , not the same ....... :-(

Have a 65” 930d that I got in December and packed up in our April move. Damn thing is still sitting in the box. Had watched a couple YouTube videos in 4K that looked great and got an ATV4 at that time. So stoked to pick up a 5th gen. Funny Apple really only had to do 1 thing for this run and they seem to have done it perfectly with the back end too.

As for the Epson, would grab another ATV to keep the Sony running wide open.

At this point Apple TV doesn't support Youtube 4K if that matters.

Will the the Apple TV support Plex 4K?

Apple TV has a fixed HDMI output to match resolution / capabilities of your display. It will play 4K HDR content "natively" or it will up-convert anything from lower resolution to match your preferred output . Obviously, native 4K HDR content ( i.e iTunes movies , some Netflix shows ) will always look better than up-converted stuff, however I was absolutely amazed by the quality of the scaler / video processing in this little box. I watched some Games of Thrones and Orville episodes from HBO GO and FOXNOW apps, while they were scaled internally to 4K HDR they looked absolutely awesome and better than their live TV versions .
So I guess to answer your question, PLEX will (may) be shown in 4K HDR, but the picture quality will depend on your original content.

But you knew when you bought the 6040 it didtn do HDR. But its a GREAT machine!

Even the espon laser 10000 doesnt have HDR. The newer 10500 does. Mine arrived earlier this week. Time to set it up. Gotta go get me a new apple tv. are they readily available now at best buy? let me check,,,,

Actually, not entirely true. First of all, 6040UB is a great machine and it does HDR, just not in 4K@60Hz with 4:4:4 chroma.
Content /picture does not have to be 4K in order to be HDR, 1080p can also be HDR. HDR is not about resolution, it's about color and contrast.
I was just upset about Epson advertising. I knew when I bought my 6040UB that it's not " real " 4K machine, what I did not know was the fact that they've used a 10.2Gbps chip instead of 18Gbps one, therefore it will not accept 4k@60hz content. As a result I had to lower Apple TV HDMI output to 4K@30hz with 4:2:2 sampling ( still HDR ) . It would be nice if I did not have to do it, since my Sony TV is perfectly capable of accepting 4K@60 with 4:4:4 color.

I have no idea the difference between 4:4:4 ans 4:2:2. I really need to take a class! lol

Will that Sony $5k unit do full HDR?

The Sony is 13.5Gbps at the HDMI board.
This means you'll get 4:4:4 12 bit, but only @ 24Hz.
Which means no 4:4:4 8 bit @ 60Hz... this is relevant more if you're a gamer than a movie fan.

Went to a Sony projector demo at CEDIA. It was 3 hours long. They showed us a bunch of material and it turns out that the most important bandwidth factor is color depth. They showed various projectors (including theirs) at 8, 10 and 12 bit depths. You could easily see banding at 8 bit. The limitation was always bandwidth. So you need at least 13-18Gbps to allow for 10 bit color or greater.

The stupid thing is that Netflix has decided to limit their 4k output to exclusively 4:4:4@60 w/HDR.

Asinine move as you cant possible stream anywhere near that quality so why expect everyone with "4k" to be able to play it?

Yes...But have they finally added Amazon Prime as promised back in June...?

I like putting in Roku better anyway...

So just to clarify is the apple tv sending out 4:4:4 if your display can handle it and if it can't it will switch to 4:2:2? I'm just trying to figure out if all of the 10.2 gb/s extenders I have put in will pass 4k from this apple tv.