A single drive is ample for 4 tuners. I have tested my 1tb ecogreen (5400rpm) with 3 tuners and simultaneous file transfer with out issue.
Reading these forums it seems that people have regular issues with raid. Raid 0 on the motherboard controller is probably your best recipe for problems.
While you may initially think you don't care about the data, it becomes a different matter when it disappears
