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.