XP can handle it.
But, did you check to see that your motherboard can? Just becasue it hase 4
slots that fit 1 Gig each does not mean that the motherboard was designed
for 1 Gig in each slot.
Also, I have seen cases in which an old program was written with some
assumption as to the maximum of RAM, disk, etc. That program may then
refuse to load or work correctly. I once even had an error that implied I
had insufficient RAM, because the program, which was win98 vintage, was
interpretting my 1Gig of RAM as some negative integer. In such a case the
solution is usually to use a newer version of the program, or some free-ware
alternative that was deisgned for XP.