Yeah 32 bit can only address 4GB maximum of RAM/VRAM.
So I don't know how it's managing it for you right now but sounds like it's not prioritising the VRAM. Get 64-bit.
Or take the unorthodox route and take out 2GB of RAM...
I run Metro 2033, Shogun 2 and Shift 2 at max settings (only use 2xaa due to rez) 2048x with no issues.