XP has a problem recognize 4 gigabyte of ram. i had 4 gigabyte in my dell 3 years ago. it will only use 3,5 (or 3,7... dont remember) gigabyte. i have seen examples where it will only use 3,2 gigabyte.
if you dont have a graphic card in your system then it will recognize all 4 gigabyte. that problem is corrected in vista 32bit where you can have max of 4 gigabyte and a graphic card.
but you CAN add 4 gigabyte. just keep in mind that most likely it will not use it all. you need a 64bit OS to go higher then 4GB (or in XP's case 3,2 - 3,7).