With the fifth RAID you have two options:
1) to put the system on the first disk of the three remaining to collect R5, built-in
2) put a separate controller and a RAID. drawback: not all controllers are to get, not all the controllers know how to create arrays larger than 2 TB.
given the volume, you have to use GPT disks breakdown, 2008 can not italianoil to boot from GPT disks — I do not remember.