the one that came with your iPhone 4 should fit. older versions of the apple usb cable had a slightly larger casing that does not fit the bumper. the cable that came with my 2nd gen iPod touch did fit however.
I'm using the cable that came with the phone. The one with the smaller white plastic on the tip. Otherwise the bumper fits well
That doesn't make sense. I have the bumper (sitting in my drawer now) but I know that the cable it came with absolutely fit with the bumper on. Did you buy the phone from ATT/Apple or off eBay?
the phone came straight from the apple store. The cable fits in the hole but does not go far enough to charge or lock in place with the bumper on.