RPi 4 4GB RAM running at 3840x2160p 60Hz on the Kodi menu. My goal was to get 4K Netflix working.
I've not been successful. 1920x1080 @ 23.976fps with inputstream.adaptive.h264.decoder(SW) on Netflix is the best that seems to be available at the moment (image quality is great by the way). 1920x1080@60fps works great on YouTube and Twitch via mmal-h264(HW) decoder.
I've done a bit of experimenting with different sources (internet vs local network) and codecs for some general understanding of the Pi4's capability. The sample clips here were super helpful Samples - Official Kodi Wiki . It seems that 2160p video (aka 4K) is not feasible on the RPi4 at the current time.
I believe in the code wizards out there (some of the sample vids via my local network were close to playable ~10-15fps). Perhaps with some overclocking and much better cooling, I might get there. Currently, I have a very basic heatsink and I'm sometimes hitting the 80C threshold. I have a new full aluminium case with heatsink showing up tomorrow. We'll see.
In summary, RPi4 and Kodi handles 1920x1080 with relative ease across different video platforms and codecs. 3840x2160 seems to be limited to the Kodi menu. Good thing my TV upscales quite well from 1080.