Re: "RAID 5 or 6"
Some interesting calculations going on there!
Pretty sure 4 x 1TB drives in RAID10 gives 2TB of usable space, not 1TB :)
Also RAID6 is better on redundancy than RAID10 as ANY two disks can fail in RAID 6 (due to distributed parity) however in RAID10 it depends:
Remember that RAID10 is just mirrored arrays (RAID1) inside a striped array (RAID0) if two disks fail from the separate mirrors, no biggie, the array can be rebuilt. If both disks are from the same mirror, you've lost all your data! (How often do two disks fail at the same time for small arrays like this anyway? And how unlucky would you have to be for both of them to be in the same mirrored array?!)
But for the performance you get over either of the other implementations it might be worth the lost in capacity of both and redundancy of RAID6 (especially if you add further RAID1 levels within the RAID0 array - 3 RAID1 arrays would mean [almost] 3x the performance of a single disk!).