Only 32 bit Windows has the 4GB limit divided between Application and OS 2/2 or 3/1.
64 bit Windows can handle 4GB per process.

Here is a rundown on physical memory support.
Windows XP Home & Pro -- 4GB
Windows XP Pro x64 Editions -- 16GB
Windows Server 2003 Web & SBS -- 2GB
Windows Server 2003 Standard -- 4GB
Windows Server 2003 Enterprise -- 8GB
Windows Server 2003 Enterprise on X86 -- 64GB
Windows Server 2003 Enterprise on x64 -- 128GB
Windows Server 2003 Enterprise on Itanium -- 1024GB

In addition there are many proprietary modifications to the Enterprise editions that extend the memory capabilities for large proprietary multiprocessor systems such as those offered by Unisys.