datahoarder
Who are we?
We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
We are one. We are legion. And we're trying really hard not to forget.
-- 5-4-3-2-1-bang from this thread
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With 4, you are correct, I went from top of my head back what we learned in high school 15 or so years ago. 5 is still better than nothing if you don't have the resources to get one more drive for 6. Of course, the best is completely mirroring all stuff to a separate geo location.
It all boils down to willingness of spending money for more durability.
I've edited my comment to scratch R4. But R5 is still great for smaller arrays, and it is possible to, for example, have RAID 5 for movies, and RAID 6 for photos.
There are also combinations of RAID levels, like aforementioned 10. There is a nice comparison table with apparent drive requirements and fault tolerance on Wikipedia: https://en.m.wikipedia.org/wiki/Standard_RAID_levels