Best practice is to build a new vdev with the 4x4TB drives and move the entire dataset across. It isn't like other systems where you can just bolt in another dr

Best practice is to build a new vdev with the 4x4TB drives and move the entire dataset across. It isn't like other systems where you can just bolt in another drive and it expands to absorb that. The critical concept here is that where other systems use physical disks, ZFS uses vdevs. vdevs are 1 or more disks but are presented to the RAID system as a single 'storage entity'. Thus, you don't add disks to a storage pool, you add vdevs.