The results are good.
Managed to get all 4 drives mounted under a RAID 5 array, like I described earlier. Once I was done swapping out the smaller partitions for the larger drives, I was left with one large partition of 699 Gigs. At least according to Yast, as well as Webmin.

However, when I did a “df”, the /data mount point still only had 331 Gigs… Anyhoo, it turns out I needed to give one final command to grow the XFS partition to the full size of the RAID array: xfs_growfs
The volume was mounted when I did this.
In my case this was:
#xfs_growfs /data

Voila, Now the system understanda that the partition is 699 gigs.

Wohoo!