From my understanding, ZFS is much better for RAID becuase it is more focused on "data integrity". Ext4 is more for an linux opperating system. I have just set up my own NAS in a RAID 0 configuration (RAID 0 because its only temporarily) using ZFS and so far, I am quite pleased. In ZFS, data integrity was in mind when they made the very first version of ZFS, thus creating a more RAID ideal environment. I often hear about the high successfull rates for data recovery or rebuilding the RAID. Overall, I would choose ZFS for a RAID configuration. Unfortunately, I cannot think of a way to be able to switch your filesystem to ZFS, I don't think there really is a way to. I hope this might help you out.