storage depends on how technical you are, and what OS.
on linux, you can use mdraid or even ZFS to mirror your disks.
on windows, it is a little more complicated, but you could use drivepool (I’ve never used, but many folks here have recommended it)
if you want something essentially plug and play and easily expandable, I would recommend a nas with 4 or more bays, the more the better (for later expansion) as much as you can afford.
finally, regarding the 4k/1080, if you are keeping separate 4k/1080, then you will not need to try to transcode the 4k files so your cpu should be sufficient, but not very future proof.
but if you meant transcode 4k TO 1080 for remote, then the cpu you have will not suffice.
for 4k transcoding you need an intel 7000 series cpu with a 600 series integrated gpu.
or an applicable nvidia card.
this may or may not have some helpful info Plex, 4k, transcoding, and you