I'm not sure what kind of calculator you're using but 2^32 = 4,294,967,296 = 4GB.
See UINT_MAX at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccelng/htm/class_34.asp

>Ken said "A full 32-bit number (32 one's in a binary number) equals 2 GB"
>
>It's actually equal to 3,221,225,472 (3.2gb). (2^0 is used in true binary.)
>
>If you want to drop down one bit (sign), then we're talking 1,610,612,736. In "micro-speak", this rounds off to 2gb.