unraid is great but on a little 4 bay mini nas with limited expandability you don't get much advantage for the money, it's better for larger arrays and lots of mixed disk sizes, and on systems where you can put in lots of SSDs to make a decently fast caching setup die to unraid slower non-striped array architecture.
On a 4 bay mini-NAS I'd go with the free truenas option and just make it a RaidZ1 of 4 disks.
For a beginner, OMV might be simpler, and for paid options, HexOS is probably more beginner friendly than raw TrueNas.
A free alternative to Unraid is Snapraid, but thats more of a roll-your-own solution, not an OS you can just install.
Yea, JF is getting mature enough for more people to transition.
I've been running it side by side with Plex for about 2 years now, and have a couple of clients (and all of my personal use) on JF, but a few users either cant run JF directly on their hardware (and don’t want to cast every time) or they are older and would struggle to learn a new app without some hands on practice with it.
The newest Plex UI update on some devices is causing some problems so I think I’ll have a few more users moving to JF in the near future.
It's a bit of a ram hog compared to plex but that's not a major issue.