Your best bet would be RAID6, 4 drives, two data, two parity drives, though the write penalty would be somewhat high, I believe it would be lower than using any sort of software raid.
Also, RAID6 provides better reduncay than anything other than true RAID11, because since RAID6 uses parity and not a combination of mirroring/striping, ANY2 drives can fail, and the array can rebuild. Also, if the controller supports it, you can expand the array at a later time by adding more drives, and expand your storage amount.
~newParadigm