Each controller has one spare, and the aggregates are both RAID_DP
I tried to create a new RAID4 aggregate using the two spares (after reassigning ownership of the other spare) but was blocked because the system said there would be insufficient spares for the new aggregate. I don't know how to 'borrow' a parity drive from the existing (almost empty) aggregate.
You still should be able to create new aggregate, may be using -f option if necessary. You can convert aggregate into RAID4 (aggr options raidtype), but I would not do it. It is not needed and RAID_DP without spares provides better protection during data migration than RAID4 + spare.