Very simple, Windows 32 bit only supports 4GB of memory per spec, and in actual use is about 3GB. Nothing can be done to change that, besides using a 64 bit version of Windows.
Your PC will support a 64 bit OS. You could purchase an upgrade to 64 bit. Your current Windows license is for the 32 bit version, so it might not accept your activation code if you downloaded the Wndows 7 64 bit ISO installation image and installed that. I've never done that, so don't know if it would work.