Win32 bit supports 4G, and xp can only support 3G. If you want to use them all, please switch to 64-bit win7, which can support 16G. How much memory a 32-bit system can use in 4 GB of memory depends on the hardware and hardware type you have installed. The more hardware, if a computer has multiple graphics cards (integrated and independent), the available physical memory will be reserved for these hardware.
You can press Ctrl+Shift+ESC to pop up the Windows Task Manager window-click the Performance tab-click the Resource Monitor button-pop up the Resource Monitor window-select Memory to view the computer's memory allocation, including installed memory and memory reserved for hardware. I hope I can help you. Thank you.