XP has a hard limit of 4GB which is a hardware limitation as well as a software one. 32 bit processors simply cannot address more than 4GB (2^32 bits) and while I believe there are some Intel-specific extensions to deal with enlarging the address space, I double checked the Microsoft documentation of the issue and all 32b versions of Windows are still subject to this limit.
While the address size increases considerably with 64 bit processors (to the tune of 18 exabytes), I'm fairly certain that 64b Windows imposes a far lower software-level limit.
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