I'd concur with the earlier post of using all the drives in a RAID 10. It'll give you a performance edge over RAID5 and with a single array you'll ensure that all the disks are equally contributing to I/O. With 2 arrays, you'd have to load balance your VMDK across the arrays and there's a chance you would end up with 3 drives doing significantly more work than the other 3. Downside is that you lose the ability to have data on one array and logs on another, but you can take care of that with more frequent backups to a network location.