In your photo there is a Phillips headed machine screw just above the mentioned 4mm allen bolt. This is the screw that centers the pads by making one arm move inor out and the other arm follows. But having said that I do as Eric S said. Loosen the nut that holds the brake onto the frame, rotate the caliper so the pads are contacting the rim equally and retighten. Pull the brake lever and see if the pads contact at the same time, repeate if not. The screw I first mentioned has a narrow range of centering and as it does that the pads will change their height on the rim. Brake adjustments are often a process of doing a bit of this , then doing that and going back to this untill all is good.