It all comes down to your intended application. You purchased 2+ TB for what reason? You need at least 3 drives for a RAID5.
RAID5 on 4 drives is tempting, but writing files will be slower on RAID5 than using a single drive. 1.8 TB effective
With 4 drives: RAID0+1, mirror of stripes (1.2 TB effective)
RAID1+0, striped across mirrors, 1.2TB effective
I would go for RAID0 on two drives with a dedicated backup routine.
Want crazy speed? RAID0 on three of 'em.
Want stupid crazy speed? RAID0 all four of 'em. Still only 5.9 in WEI I think.