4G Memory for XP Problem

I just install 2 X 1G memery into my PC win XP Pro. The system only show 3.25G memory(I had 2 G before). If I install 3G, it show 3G. It looks like the XP can only reach to 3.25GB memory usage. I did some research online, it looks llike XP can only handle upto 4G memory.

Can anyone know why my XP only have 3.25G even I know I installed 4 G memory. Any way to fix it?

thanks