Firestick 4K Max is new hardware. I don’t think the Plex app has been really worked over yet to function best with it yet. There’s been a couple threads posted here on issues, and I remember similar threads about the FireTV Cube when it first came out.
I don’t know about the Chromecast/Google side of things much but there have been some threads about Plex not being able to use some audio formats and I’ve seen mention its because the hardware doesn’t identify itself easily. So you end up with the app assuming the entire class is lowest-common denominator, since it can’t tell you’re using a higher-end model.
I’ve never really looked into Chromecasts as I’m not interested in using my phone as a controller. Once the new one came out that’s more like a traditional streaming dongle I already had two Fire Stick 4Ks and a Shield Pro.
