You cannot use the iPhone 4 dock with the bumpers on. One solution is to use the universal dock without an adapter. This is not an optimal solution, but works.
Hope Apple makes an adapter specifically designed for the iPhone 4 bumper.
But for now, I decided to build an adapter by myself. It's based on the one for 5th Generation iPod (No. 9). I cut the bottom part off and changed the shape a little bit.