So you created a true/fake hardware RAID with this card and initially 3 disks?
When it's possible at all, it must be done on a RAID controller level, as the data needs to be re-arranged. Either when you reboot in the BIOS of the card, or in their MegaRAID Storage Manager application. Itís based on an LSI card AFAIK, and some support to extend the capacity (and others are only fake RAIDs?). Is there something in the documentation of the card or its software about it?
When you got this done, you have to use something like gparted
from a Live CD to adjust the partitions.