Why do some A380 main wheels have no brakes? The A380 has 20 main and 2 nose wheels. Apparently the nose wheels have no brakes (compare with Are there any aircraft with a nose wheel braking system?).

While researching for this question (How are brakes cooled on heavy aircraft?), I assumed that all of the 20 main wheels are braked. I looked at a few pictures of A380 landing gears. As far as I have seen, all main wheels are equipped with brake cooling fans (BCF), except the four rear wheels in the inner 6-wheeled  undercarriages. (e.g. here).

This picture below clearly shows that there is no brake unit inside the rim:



There is another picture here whithout BCFs, but the same four wheels apparently have no brakes.

What is the reason for omitting the brakes at these four wheel positions?

Is this a common configuration, or quite unique to the A380?
Because they are not needed (for the passenger variant). Addition of brakes increases the weight and complexity of the system, which is undesirable. Why add something that is not required?. If it is required, additional brakes can always be added, like in the (proposed) freighter version. According to flightglobal.com:


  Sixteen of the A380-800's 20 main wheels are braked, with the two aft wheels on each body landing gear of the passenger version not having brakes (unlike the freighter variant, where all 20 main wheels are braked due to its increased weights). 


Also, Aft Axle of BLG is steerable to ensure better maneuverability for small radius turn (Body Wheel Steering); braking a steerable axle would require special linkages, increasing weight and complexity. This is not insurmountable, however- Boeing 777's landing gear is similar, but has brakes on all its wheels.