Windows XP or Vista 32-bit edition can not use 4 GB, there is nothing to do, it is a mathematical limit.

32-bit system can address:
2 power 32 = 4.3 GB addresses
... As it needs to send something other than the RAM (resources motherboard, expansion cards, including the graphics card), it remains only about 2.8 to 3.3 GB of RAM addressable following configuration PC.
In the end only 2.75 to 3.5 GB of memory is actually usable, and this, by the very design of the x86 memory architecture.
To note that the exact amount of memory that Windows XP recovery depends on the chipset and BIOS on the motherboard.
This difference between the amount of physical memory installed and the amount available under Windows due to the presence of an area high in the memory.
Located just below the 4 GB, which is stored the memory used by the various devices PCI expansion, something imposed by the x86 architecture.
Therefore, the onboard video memory for graphics cards has just cut, in part, the total amount of memory available ...

It is therefore not a breakdown or a bad setting in your PC

I draw your attention to the last sentence, the system memory, graphics will be deducted from the RAM installed:
If you have a graphics card with 2 GB of memory, or CrossFire 1GB card, Windows will not see more than 2.8 GB addressable, with a 3-way SLI GTX 280 of the value of memory for Windows falls 2.5 GB

What can we do?

There is a trick, the PAE (Physical Address Extension), to address the memory of 36-bit OS with a 32-bit, but it does not, unfortunately, in practice, since Microsoft has disabled this function for reasons of compatibility drivers and applications general public:

"... even when the parameter PAE is enabled, the amount of memory available in Windows is still less than 4 GB with the same 32-bit edition of Windows Vista"

"to avoid potential compatibility problems of pilots, 32-bit versions of Windows Vista limit the total available memory to 3.12 GB ..
If a computer has many devices installed, available memory may be reduced to 3 GB or less. However, the maximum memory available in 32-bit versions of Windows Vista is typically 3.12 GB "

On a more professional system closed, you do not need to ensure compatibility of thousands of drivers, it can activate an EAP or something equivalent.

Conclusion

There is no way to use more than 3GB of RAM on a Windows 32-bit. The solution to use 4GB or more on a PC general public is to switch to 64-bit Windows, taking care that all drivers and applications do not yet exist in 64-bit ...