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1080p should be fine but 4K is not possible without an OS that supports HDCP on the video pipeline. There is development in that direction for Linux on Intel hardware but exactly what hardware ends up supported and when it happens are total unknowns (not anytime soon).
Thank you for the answer. At the moment I'm on mecool m8s pro w with coreelec, but I can watch Max 720p and even with that sometimes stuttering happens... (Especially on "our planet" more complicated scenes). I'm thinking about some interesting based fan less pc like this one or gigabyte brix (Celeron n3160) but I don't know if it will be any better.
If anyone could confirm that I could run Netflix @1080p without stuttering on either of this PC and libreelec would be great.