Amazon has the Western Digital Red WD40EFRX 4TB 3.5" Internal SATA Hard Drive for a low $120.66 Free Shipping. This sells for $140 at NewEgg, so you are saving almost an extra $20 off.

WD Red hard drives are designed and tested for compatibility in the unique 24x7 operating environment and demanding system requirements of home and small office NAS. With exclusive NASware technology built in, WD Red hard drive improves NAS storage performance by reducing common hard drive concerns in NAS systems.