Why Windows is not detecting the 4 GB of RAM installed?
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 to the power of 32 = 4.3 Giga different addresses, corresponding to 4.3 GB of RAM
as it needs to manage other stuff than the RAM (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 due to the design of the x86 memory architecture ...
This difference between the amount of physical memory installed and the amount available under Windows can be justified by the presence of an area high in the memory.
Located just below the 4 GB is stored, the memory addresses 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.
What can be done?
There is a trick, the PAE (Physical Address Extension), is needed to address the memory of 36-bit OS in 32-bit, so it is theoretically go up to 64 GB of RAM 32-bit
but unfortunately it does not work well for general purpose PC:
The reasons:
In order to avoid potential compatibility problems with, 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.
Note that:
Microsoft disables by default this PAE, and recommends not activate it for compatibility with drivers and general public applications:
"Some drivers may not load if PAE mode is activated ..."
"Other drivers can load in PAE mode but cause system instability ..."
On a more closed professional environment you do not need to ensure compatibility of thousands of drivers an the PAE can be activated.
Conclusion
There is no way to use more than 3GB of RAM on a general purpose PC with Windows 32-bit edition.
The solution to use 4GB is to switch to 64-bit Windows version is the recommended solution by Microsoft.
But most of the drivers and applications don’t exist in 64 bit version…………..
