You're putting so much high quality hardware in here I would really like to urge you to try and find a deal on a set of enterprise rated 4tb disks instead. For instance I should think with a but of patience and/Or a strategic coupon you could find these disks for closer to your price range. These are rated for much higher duty than you'll be using them for and should increase the useful length of your disk set substantially.

http://www.amazon.com/HGST-Ultrastar-3-5-Inch-Enterprise-0F14683/dp/B007VL8WU6

I didn't nit pick hard disks originally because I'm honestly not so sure it's a big deal either way. I'd probably go one way or the other though, find cheaper disks closer to $80 - $100 each even if you've got to bust them out of external enclosures or go all the way to the enterprise class, I guess that's my real advice is to not get stuck in between with these NAS disks which I am more or less convinced are the same disks as the lower priced ones with different firmwares. Don't get WD greens though. They are trouble. (specifically for NAS use, I mean, due to aggressive powering down)

Finally, with regards to hard disks and your data. Don't forget that infant mortality rates are fairly high as well and be sure to give your server a good week or two of testing to be sure it's ready for your production data especially if it's going to be your only copy of some media. I won't do more than mention in passing to research the 3-2-1 backup method.

Tldr; go enterprise or go cheap, and don't trust them immediately out of the box.