Durability of RAID-Z3 vs 4-way mirror

Both RAIDZ3 and 4-way mirrors can lose up to 3 drives without data loss. However, you must consider that RAIDZ3 will have much lower read IOPS and higher resilver time after a disk replacement.

Anyway, I really think 4-way mirrors is way too much (other things will broke before four drives die). If IOPS are not a concern, I would use a RAIDZ2 array.

That said, if you are sure that no additional space will be required, for a 4-bay system capable to sustain 3 disk failures, I vote for 4-way mirror rather than RAIDZ3.