...as soon as I know find out is XP Pro will recognize a 4GB stick.
The Per-slot maximum, is handled by the BIOS which handles how the memory is accessed, not the OS. That said, there have been boards that when populated with 2GB modules do not boot some OS's.....this is not the fault of the OS, but rather the BIOS. If the OS was
the cause, then NO mobo populated with 2GB modules would boot, and that simply isn't the case. There should be no reason, if your mobo BIOS recognizes 4GB modules, for XP-64 not to also recognize the memory.