Google has updated the design of the Nexus 4 with a slight hardware modification to help with a common problem. Many users have complained about the slickness of the device when set down on a surface. While the bumper that Google sells helps with this, it makes the device bulkier and not everyone will accept that.
As you can see in the picture above, there are now little rubber nubs around the perimeter of the device. These help raise the device up and off of the slick back. They have the added benefit of helping the rear speaker sound a bit better and louder when the device is laid on a table. Finally, they help the device avoid being scratched.
Ultimately, the nubs are noticeable when holding the device. But, they are likely better than the bumper. If you pick up a new Nexus 4, you'll likely have the new hardware revision with the updated nubs.