I really hate to be a ball buster here, but at what point did you associate Raspberry Pis and "high performance NAS." Although it'd be a cool project like this, Raspberry Pis just aren't that powerful. They're low cost for fully functioning computer, but they are by no means cheap for their computing power.
I would recommend a device more adequately designed for the purpose you have in mind. A typical consumer NAS wouldn't cost much more than the 4 RPis ($140 + S/H), the HUB you'd need to interface them, and the 4 power supplies (or one bigger power supply) you'd need. In addition, you'd need the 4 x 1 TB drives you mentioned, all with USB enclosures (RPi doesn't have SATA). All these together would run over $450 easily.
Personally I use an Apple Time Capsule, as it acts as a wireless NAS, printer server, 4 port ethernet hub and dual-band wireless N router all in one. It's $300 for the 1TB model or $500 for a 3TB model, although you can easily swap out the drive of the $300 model to your own 3TB drive for less than the $200 cost difference that they charge.
If you require a RAID array for data security/safety, something like a Drobo might be worth looking into.