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