Two Mirrored arrays, or one RAID Z, or RAID Z2?

Ok. Here's something I'm trying to find out. I have a NAS box with 4 drives. And it's set up as two RAID 1 arrays that I access over my network.

Storage space aside, which is best for data integrity? Mirrored or RAID Z or RAID Z2.

And the only reason I ask is because Logan had a mishap recently with his system. From a little research it seems RAID 1 and RAID Z2(RAID6) is safer than RAID Z(RAID5)?

So using 4 drives, what is the safest RAID level to use, totally disregarding the usable storage space?

Thank you to all that help ye!