Let's let you pick the answers from this link!
-> http://www.google.com/search?&q=XP+SHOWS+LESS+THAN+4GB+RAM
It's been asked a few times.
Bob
I just added 4 gb of Ram, (2 cartridges of 2 gb each), of DD2 PC5300, from Crucial. While the computer does run much, much better, I noticed when I went to SYSTEM in the CONTROL PANEL, that the machine is only recognizing 2.99 gb of the new RAM. Why is that? Does it make any difference in performance, and if so, what can I do about it? Thanks for any help or input you can give.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic