it might be a motherboard limitation - some dont have the capacity for more ram.  mine is limited to a total of 8gigs, and its maxed out right now - i'd love to shove a few more sticks in there, but there is no room. 
i also run xp 64 pro.

edit:  side note, it doesnt matter if i use two sticks that equal 8 gigs for mine, or 4 sticks of 2 gigs each, my cap (top limit) will remain 8gigs because thats all my MotherBoard can handle.

this might be your own limit - 4 gigs no matter how you configure it.

Rarer than a hairy egg and madder than a box of frogs....

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