This is enabled via the PAE (Physical Address Extension) extension of the PentiumPro processors. PAE addresses the 4 GB physical memory limitation and is seen as Intel's answer to AMD 64-bit and AMD x86-64. PAE allows processors to access physical memory up to 64 GB (36 bits of address bus). However, since the virtual address space is just 32 bits wide, each process can't grow beyond 4 GB.
