This is really quite simple. It is an iPhone 4 with white replacement parts he ordered online. Now, the iPhone definitely started as a black iPhone 4 but could've been swapped out with real white parts from a site like whiteiphone4now.com. I would recommend getting a black iPhone 4 and ordering the service yourself just so that you know exactly where the phone has been plus you get the original black parts just in case you need to take it back for a warranty exchange. There are a number of places online where you can order the white parts and it's pretty easy to change the parts yourself (takes about 5 minutes and much less than $100 if you DIY.) Note that the screen will cost more than just the back.