Well, its time to draw some conclusions.
The real contenders here are RAID 5 vs RAID 10, as the other RAID levels have big cons that exclude them from many servers (especially RAID 0, that is too prone to lose your data).
So, who is the winner between RAID 5 vs RAID 10? Obviously, it depends: sometime RAID 5 is faster than RAID 10, but many times we have the opposite picture. So, I generally like RAID 10 much more than RAID 5, as it wins many tests and give me added resilience against data loss.
However, there is a catch: RAID 10 costs you more disk space than RAID 5. While the former ask you not less than 50% of yours disks space, RAID 5 only want a fraction of your total disks space. For example, in a 16 x 1 TB disks systems, RAID 10 left you only ~8 TB, while RAID 5 give you a whopping ~15 TB. So, with increasing number of disks, RAID 5 is in a better light.
Nevertheless, if the lost space is not a problem for you, I warmly recommended you to run your software-based RAID array in a RAID 10 layout, as the added performances are a very healthy addition. From a performance standpoint, to be really competitive RAID 5 need to be paired to high-end dedicated hardware controller with plenty of cache memory: in this case, the controller's cache subsystem can effectively mask the latency involved in the read-modify-write behavior and can also try to fuse multiple, small adjacent write requests to one single and bigger write transaction.
To recap.... use RAID 10 when:
it is not a problem to lose 50% of total disks space
you need the added performances guaranteed by this layout
you use a software-based RAID implementation
Instead, you can use RAID 5 when:
you have an high-end hardware-based RAID controller with on-board cache memory
you can not afford to lose 50% of total disks space
you don't have to run an heavy, transactional database on the RAID 5 array
So, in the end, for the very common 4 disks, software-based RAID setup I recommended you to use a RAID 10 layout.
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