What is the point to run RAID6 on 4 drives, instead of RAID10? With both, half the space is usable, but with RAID10 read speed is twice as high, and resilvering is twice as fast. RAID6 makes sense for 5-8 drives.
Actually, according to recent testing by ArsTechinca [0] performance with RAID the topic is more complicated. The testing is focused on ZFS but, quoting from the conclusion: If you're looking for raw, unbridled performance it's hard to argue against a properly-tuned pool of ZFS mirrors. RAID10 is the fastest per-disk conventional RAID topology in all metrics, and ZFS mirrors beat it resoundingly—sometimes by an order…
Emphasis on 'properly-tuned'. This doesn't hold as true as the OP states for the defaults (which is what the majority of home users will be using)