Lots of AndroidTV media players and normal Android media players are advertised as 4K.
But video playback is actually downscaled a lot or even to 1080p in some cases.
This is most noticeable in apps like Kodi, VLC or stock video player apps if playing local stored content that are high bit-rate 4K or 4K@50/60hz.
For AndroidTV Netflix Certified devices
Apps like Netflix will most likely not do downscaling since most content is 4K@23.976/25hz but there will still be some line skips, not all 3840 horizontal lines will be displayed every second since the S905x processor is not powerful enough, it depends on the bit-rate and frame-rate, but most people won't notice this.
To check if your device plays back true 4K(4K@50/60hz) instead of downscaled 4K, people can follow this guide to determine if their device supports true 4K video playback.
https://mrmc.tv/forum/viewtopic.php?t=4060
The test video is provided by the lead Kodi Android developer Koying that retired and work on Mrmc the AppleTV Kodi app now.
To play back 4K@50/60hz and/or in 10-bit you also have to use an HDMI 2.0 cable since an HDMI v1.4 cable bundled with 99% of devices only support up to 4K@24/30hz in 8-bit and also set your tv to enhanced/deep color mode.
If your device supports auto frame-rate switching in Android Kodi or in CoreELEC/LibreELEC then the video will play correctly otherwise it will be downscaled. If you use an expensive tv then the tv will sometimes also do it's own upscaling again to improve things.
It would be interesting to know which devices support true 4K(4K@50/60hz) and which ones only support basic 4K(4K@23.976/24/25hz) with downscaling occurring in 4K@50/60hz because some of the horizontal 3840 lines have to be stretched because some are skipped to compensate for the weak CPU/VPU in S905x devices.
---------------------------------------------------------------------------------------------------------------------------
Apart from the Nvidia Shield and Xiaomi Mi box, 99% of other Certified AndroidTV S905x devices don't support auto frame-rate switching, so all your 1080p/4K@23.976hz videos will play back at 60hz instead. This will result in audio and video becoming out of sync. Some people also call this stuttering then the video might freeze for 0.5-1 second and then resume again to get the audio in sync, happens about 3-4 times in an average movie of 1.5 hours.
Using a device with auto frame-rate switching or an Android media player with LibreELEC/CoreELEC support(which Certified AndroidTV devices don't support) then audio and video will play perfectly in sync and video playback will be much smoother.
If someone has a lot of 1080p/4K@23.976 or 1080p/4K@29.97 content and not using LibreELEC/CoreELEC or an Android device with Kodi auto frame-rate switching, you are missing out a lot on smooth video playback especially on a 4K tv.
If your tv has a 120hz instead of 60hz panel then it also helps a lot to reduce this stuttering.