I ordered a white iPhone back from a seller on amazon. Cost me $15. In hindsight, I should have used black, but ohh well. Make sure you get an authentic Apple iPhone 4 back, too.
Step 1: Remove the Glass From the Plastic Bezel
Step 2: Divide the Back Into It's 2 Parts
Remove the glass (left) from the bezel (right).
Step 3: Remove Interior of Bezel
I used (a bad choice) micro scissors and a razor to remove the inside of the bezel. All that should remain is the plastic around the outside. Keep in mind that the small hooks have little metal pieces running up them, so don't just pull them out, or you'll ruin that hook.
Step 4: Remove White/Black Paint
Now you need to remove the paint from the glass. I started using paint thinner, then realized a razor blade worked MUCH better. The apple logo and text will not come off with the paint thinner, but if you're using a razor blade, be gentle, or you'll scratch a bit off, like I did.
Step 5: Adhere the Glass to Cleaned Bezel
I used a glass glue, and it held just fine. One thing to note, DRENCH the bezel with glue, have it overflowing on all sides, then use the razor to trim it back once it's dry. I did not do this, and I can see glue drops through the glass.
Step 6: Attach Back
Remove the two bottom screws, and slide the back of the phone UP. Put on your back, and there ya go!