We were already very much convinced about the qualities of the Red disks from Western Digital we tested earlier. The same holds for de 4TB model. The disk definitely isn't the fastest around, but that's not what is sets out to achieve in the first place. It does, however, consume very little energy, which is very important for disks in a NAS device, which may be on 24/7. Noise levels are also well within the margins of what we consider to be acceptable, even though other disks in the Red line-up were even quieter.
A definitive conclusion about the Western Digital Red 4TB is difficult to draw, as we don't know the price of the desk yet. If it's going to be priced along the same lines as the 3TB model, we would certainly recommend this disk if you are looking for disks for your NAS device.