Because your machine only support 4GB?
Honestly, in Apple 's world. Customer aren't suppose to know the tech wiz bit, if 4Gb is what they say they will support, 4GB it is then.
Inreality it is not only OS problem. You need BIOS as well as proper OS settings.
X86 32Bit supporting more then 4GB of memory will require PAE ( Physical Address Extension ).