That's normal because the 32 bit version can only use around 3.7 GB of regular ram + video RAM. My macbook has 8 GB ram and a nvideo 9400, i guess 256 mb video RAM, and only see 2.3 GB. Apple has some other limitations that take a whopping 1.4 GB of integrated video RAM.
64 bit windows will see and use the full 8 GB.