Please refer to the Disk Planning Guide
for further information.
In general, you cannot convert a 2-drive RAID type into a 4-drive RAID type. You will have to remove all data from the existing volume and then recreate the system using 4-drives.
As always, it's recommended to use a Redundant RAID Volume, using JBOD/RAID-0 are the most vulnerable RAID types - therefore it's not recommended to use these RAID types.
Hope this helps