I assume by system properties, you mean from within Windows. Which version of the OS are you using, and are you using the 64-bit version? You need 64-bit Windows to use more than 4GB, so I'm guessing this is your problem.
I'm not on my Win7 machine now, but Win7/Vista should tell you x86/32-bit or x64/64-bit with the OS version on the system properties page. If you are using XP, you have to have Win XP Pro x64 edition.
Nothing else in your system should be limiting your memory usage, so I'm just guessing you have a 32-bit OS installed.