Thank you for the clarification. So the most common Windows versions (XP and Vista) actually don’t support PAE to access RAM beyond the 4 GB limit, due to concerns about driver compatibility.
That explains why I only see 2 GB under XP when telling my BIOS to remap memory around the PCI “hole”, even though Memtest can see all 4 GB. So PAE is really a nonstarter for most readers of this blog.