In order to get the entire 4GB to be recognized and used by your system, you should use a 64-bit Windows. As far as I remember, if you're using a 32-bit operating system, it'll only show you have (and use) only 3.25GB. Actually x86 is an instruction set architectures based on Intel CPU, launched in 1978 as 16-bit extension of an older microprocessor. Since then, new, faster extensions have been created, x86-32 bit, and later on, x86-64 bit.
So my advice is, if you're going to use 4GB RAM or more, go with the x86-64 bit operating system. If you want to read more about x86
bit, and x86-64 bit
, click those links.