|well, a mirror is RAID. it's just not RAID-5, is that what you were asking about?|
if you want the FASTEST performance, you'd want RAID-0 where 2 disks are joined to create a single partition. Most people RECOMMEND AGAINST this because if EITHER HDD dies, you lose data from BOTH partitions. So you double your chances for data loss (or at the very best, you've doubled the chances that you'll be rebuilding and restoring from backup).
mirrors are Raid-1, where the same data is written twice. This is a safe way to go because if either HDD dies, the other HDD still has 100% of the data. We have a server with a Raid-1 OS and just about once a week the array degrades and we need to re-attach the second HDD. This can all be done with the server up and running, and the mirror begins to rebuild automatically from the existing data on the good HDD. Theoretically there is next to no performance hit because each HDD can write independently of the other so they can write at the same time.
The best combo of performance and fault-tolerance is Raid-5, where 3 or more disks combine to give you increased storage AND the ability to rebuild all data with the loss of 1 HDD. For example, if you had 3x500GB HDDs, you could set them up in RAID-5 for a single 1TB partition (one of the disks is used for backup, kinda...). Add a fourth 500GB HDD and you'd still only use 1 disk for redundancy, so you'd have 1.5TB of storage.
To increase performance in this scenario, you could create multiple PARTITIONS on the single RAID-5 Array: So if you took 3x500GB HDDs and made a single 1TB array, you could break that down into a 250GB OS partition and a 750GB Data partition.
all that being said, there is really nothing wrong with the setup you've described (4 HDDs, 2x2 mirrors). You'll have great fault-tolerance and no real performance hit.