Is this powerful enough to use as a Plex server?

Streaming 4k HDR has a lot more to do with codecs used and the hardware deciding capabilities of the clients. So for the server you just need minimal disk speed and average network speeds if your client devices are good. If you do need to transcode you will want an Intel processor that has quick sync. But I would avoid transcoding (waste of electricity) and rely on clients with good hardware decoding. Hevc, av1 etc. My old Nvidia shield device didn't support hevc and had to replace it avoid transcoding my movies.

This is a solid decoder, cec support sucks but most Android TV devices do. Anything with a Amlogic S905X4 or newer should support everything https://a.co/d/as1T5RY

I picked up the basic N95 equipped model to run as a little VM box and replace some Raspberry Pis, and yes it is most definitely enough to be a decent little plex server.

The alder lake iGPU can handle 4K transcoding to multiple clients at once, though I still recommend avoiding transcoding wherever possible. if your main playback device is the shield, you will very rarely, if ever, need to transcode.

I havent done any longer term testing as I use a larger purpose built system as my server, this little box is mostly running homeassistant and some linux VMs

Upgrade your Plex box, but keep that Nvidia shield! It's still one of the best Plex clients on the market and will direct play almost anything.

You could probably get by with something cheaper for your replacement. I found a local guy on craigslist selling these and got one with 32GB of ram and a 128GB m.2 for $175.