I Found a problem/bug, Ok first the setup.
- If you hold Select & Down Arrow (at the same time) for 5 sec. then release and push the menu button a dev menu pops up.
- Enable System X-ray (it will show you system stats)
- If you look on the top left you will see where it shows you what mode the fire tv is currently in
Ok now the problem, from what i read on the forum is that fire tv dont switch to 4k mode unless you have a 4k h.265 movie playing, from what i found out this is false. If you open youtube app, as soon as you open the app you will see that the fire tv switched to 4k mode (but when you open plex it stays in 1080 mode). So saying that the only way to get to 4k mode is to play a 4k h.265 movie is not true. If plex program the plex client to start in 4k mode even if you play a 1080p movie/show/whatever it will be played in 4k mode (test it open youtube and play a 1080 video it stays in 4k mode it dont switch). So people like me who have 4k movies in H.264 format, if fixed, will be able to watch them in true 4k. Would love to hear back from a dev about this or anyone else.
Edit: Reference https://developer.amazon.com/docs/fire-tv/4k-apis-for-hdmi-mode-switch.html
WRT H264 4K Video:
Your 4K/H264 media may be exceeding the capabilities of the AFTV.
The AFTV Gen 3 & Cube support 4K H264 video up to High@4.1. The AFTV Gen2 supports 4K, but only for HEVC. See Tech Specs.
If your media exceeds the limits of your device, it will be transcoded by Plex. Plex currently transcodes 4K material to H264 1080p.
Note: I’m not aware of any H264 4K material that fits within High@4.0/4.1. If you look at the H264 profiles (Wikipedia), High@5.1 is the minimum for 2160p@30fps.
a) Examine your media with MediaInfo or Plex XML. Verify the H264 level & profile.
b) Compare against capabilities of your AFTV (link above).
c) Check with Information Overlay. In Plex client on AFTV, set Display Information = On. During playback, pull up on screen controls (pause, etc) and look in upper left corner. The status will show if Plex is Direct Playing / Direct Streaming / Transcoding the video & audio. If transcoding, it will provide a reason.
I have a AFTV Gen2 box. It supports H264 1080p up to High@4.0. When I play a H264 4K High@5.1 file, Plex transcodes the video 1080p. Information Overlay reports the video is transcoded because it is not compatible with the device. The On Screen Display for my LG TV also reports 1080p resolution for the video. In my case the transcode to 1080p is expected, as the AFTVg2 does not support 4K H264 video. This is just to show that the Info Overlay can help with troubleshooting.
WRT YouTube / Netflix / Amazon Video:
They are not using H264 for playback. YouTube uses VP9 for 4K playback. Netflix and Amazon use HEVC for 4K playback.
WRT HEVC 4K Video:
I am also having trouble with this.
Plex is playing my 4K HEVC non-HDR videos at 1080p. The AFTV g2 reports Direct Play. However, my LG TV reports 1080p (it reports 2160p when playing 4K Netflix via AFTV app).
I have not yet dug into this further (looking at logs, etc), but on the surface appears to be an issue with either Plex client or ExoPlayer2 (the underlying Android video player used by Plex).