I have a WD sata drive that I want to put in an external USB enclosure. Physically, the drive is 4 inches wide. I know about 3.5 inch and 5.25 inch enclosures, but what do I use for this. It's a WD5000AAVS, spec here: http://www.wdc.com/en/library/sata/2879-701229.pdf
Consumer hard disks nowadays usually come in two forms: the 2.5" and 3.5" form factors. The 2.5s are usually for laptops, while the larger 3.5" you'll most likely find on desktops.
The WD hard disk you linked to is a 3.5" hard disk. Hard disk enclosures are always advertised "for 2.5" hard disks" or something similar. So, look for an enclosure that can support a 3.5" hard disk.