I run Kodi/LibreElec w Jellyfin plugin on a rPi4-8gb with everything mounted via SMB.
Successfully watch 4k content no problem.
Your biggest problem/concern ends up being hardware decoding. You want something that can do x264,x265-10bit,etc. DD/Atmos decoding is way less intensive.
Just check your CPU/GPU for hardware decoding support when you do a purchase as that is where you'll want to splurge.
Any new 2020+ Intel nuc/mini PC will be over powered for this.