RAM Upgrading Problems

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

32-bit system can only address:
32² = 4.3 GB. Different addresses, corresponding to 4.3 GB of RAM as it needs to manage other hardware and not just  the RAM but such hardware like the (motherboard, expansion cards, including the graphics card), therefore only around 2.8 to 3.3 GB of RAM is addressable depending on the PC configuration.

In the end only 2.75 to 3.5 GB of memory is actually usable, and this is 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 would have taken it's share of the total amount of memory available. 

So what can you do?

There is a trick, the PAE (Physical Address Extension), is needed to address the memory of 36-bit OS in 32-bit, so it can theoretically go up to 64 GB of RAM.  However 32-bit unfortunately does not work well for general purpose PC:

Reason?

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

Take note:

By default Microsoft disables  this PAE, and recommends not to activate it for the reasons of compatibility with drivers and general public applications because, Some drivers may not load if PAE mode is activated and although other drivers can load in PAE mode but may cause system instability.

Conclusion

You simply can't use more than 3GB of RAM on a general purpose PC with Windows 32-bit edition.

Solution 

If you desire to use more than 3GB, then switch to 64-bit Windows version as recommended by Microsoft.