Hello,
From the verbiage of the process itself, NetApp claims that an option 4 has the following result:
Zero disks, reset config and install a new file system?: yes
This will erase all the data on the disks, are you sure?: yes
Rebooting to finish wipeconfig request
That said, I've not been able to find a clear explanation as to how much of an effective wipe this applies vs. the disk sanitize function. I would suspect that a 7-pass sanitize is more "complete" than the wipeconfig, but as you observed, the sanitize can only be executed against spare disks. So whatever disks make up your vol0 obviously can't be part of the sanitize procedure.
Maybe a wipeconfig guru can give us some more information insofar as if that procedure zeros things out as effectively or not. In the meantime, you could run the 4a - rebuild the root aggregate with 3 disks - boot back into ONTAP and then apply the sanitize license and sanitize the remaining disks (which are now spares). It's a bit of extra time - but I don't know how else you'll be able to run the sanitize command...
Good luck,
Chris