I don't know for certain, but I think you'll ultimately have to decide between either low power consumption or 4k transcoding. I doubt you'll be able to achieve both.
It's pretty sweet. I went the other way, starting on a Pi 4, moved up to a cheap ($110) Celeron N3350 device, then finally this little beast when I started getting HDR content and needing to transcode with tonemapping. 4 times the RAM, double the cores and it's just way faster in general.
I haven't personally opened it up, but it does internally use a replaceable m.2 NVME SSD according to the info that came with it, so you should be able to.
Yeah, in my case I host my hard drives on a cheap ARM Synology NAS and an external drive plugged into an M1 Mac Mini running Asahi. Just have an external SSD plugged into the Jellyfin server as a cache/transcodes drive