Even 32-bit XP does not deliver 4.0GB. You should see it deliver from 3.0 to 3.5GB since some of the 32-bit address space is used for the video and other spaces.

But let's move back to reality. Today your best machines are running on 2.0 GB. I've yet to find a real reason to go beyond this.

There is also that 2.0GB Application Memory Limit in 32-bit XP (research the /3GB boot.ini switch.)