From the article there is nothing that would make it seem that the 4G would not be available using CUDA, though there is a hint there may be more wait states involved when accessing all of it. Someone with this card and the CUDA devkit installed could verify that.
All of the memory is accessible. Memory read performance is worse than on GTX980 on the first 3.5G and the performance from the last 512MB is atrocious.
