cudaMalloc problem on a Titan card

I have a gtx titam card with 6GB memory. But I failed to malloc 4G consective memory. Two 2GB memory is OK. Why?
My OS is Win7 x64
CUDA version is 5.5
Driver version is

Can nVidia please do something about these spambots?

The WDDM driver model used by Windows Vista and later versions imposes restrictions on the maximum memory allocation size.

Adding extra system memory to your PC might help a bit. A switch to Linux would fix your problem instantly.

Thank you very much for your reply!