Looks like the normal limitation of 32 bit windows operating systems.
Google "4gb ram 32 bit windows". (The bad news is that you'll get more than a million hits.)
The details of your hardware determine the amount of free memory available to applications, so there is no standard number for a 32 bit OS with 4GB of memory installed.
If that annoys you too much, the solution is to use the X64 version of the OS. It cannot be installed as an upgrade over a 32 bit OS, and it precludes running old 16 bit (DOS era) software.