In my experience with the latest nightlies an RPi4 can handle 4k60 H.265/HEVC/HDR videos fine, but it struggles with 4k60 H.264/AVC and quite a few other codecs like VP9 and VC-1 etc. What I'm trying to say is that an RPi4 has potential, but it's still a bit rough around the edges and nothing I'd consider running 24/7 in someone else's home yet.
Compared to an S905X3 box running CE, it's no comparison really, that SoC will chew through anything you can throw at it pretty much.
Don't overlook something like an Nvidia Shield TV, think about how it's going to be switched on/off and general ease of use etc.
that's what I was afraid of with the rpi4. Thanks for confirming. I will look for an S905x3 box. I had considered a shield, but it would be kind of a "waste" as this thing is just gonna be an offline local media player. I like the idea of a simple gui with Core or Libre that goes straight into the content.