If you're after speed then RAID0 is the way to go as this stripes the data across the drives so effectively it sums the performance of the drives. That's the theory, real-world figures will be lower but still significantly higher than using the drives singly.
<added after a bit of thought>If you're going to buy four HDDs anyway and if your mobo will handle it, give RAID5 a thought. This splits the data across all the drives so read performance is very high and you also get resilience in case of a drive failure. According to Sandra, 4 drives in a RAID5 on my DS4 mobo was 2.5x the performance of a single drive.
Jonathan