Hi.
4 disks means you can use different types of raid:
RAID 0+1 = 1800 GB usable
RAID 5 = 2800 usable
RAID 1 + RAID 1 (2 disks + 2 disks) = 1000 GB + 1000GB
Said that, I assume they are 10k disks.
Depending on what you want to do, creating 2 partitions is not the best idea. On you C:\ drive you will have Windows & programs, do you really need 1000 GB?!?! This can be taken as example:
FIRST RAID 1 --> part #1 = 300 GB - Windows / Programs
--> part #2 = XX GB (I don't know how much do you need) - E-mail and documents
--> part #3 = XX GB (if you still have some space...) - Backup
SECOND RAID 1 --> part #1 = XX GB - Database
--> part #2 = XX GB - Backup / other
if not you can use a RAID 5 in a similar way, with the first solution you will have better performance because of the 4 disks are separated in 2 groups of 2. No matter if you plan to use exchange and SQL or others software... both e-mail and database are DATABASES and use disk... it's a good idea to keep them separately.
If you need help... I'm here :)