The speed on a CDR is the speed that it is rated at for burning. If a disc is listed as 4x, you can try to burn it at 12x, but it might not work. In my experience, the drive will try to burn a disc at the highest possible speed for the drive, unless you set the speed manually. The rating on the disc only shows what speed(s) it is guaranteed to burn at. I've heard of people burning 2x rated CDs at 8x, so I don't see why giving a 4x CD a shot at 12x would be a problem.
-Tom