Don't know how it happened, but it was pretty painless!
2 - 5 minutos
I keep my iPhone 4 in a BookBook case, never drop it, and I don't recall anything hitting it while in my pocket and the back panel cracked!....
.... But fixing the issue was no problem. Ordered the new back panel, 2 days and 2 screws later it was as good as new!
Be careful! :)