The MMU on some 32-bit processors can actually support more than 4GB of physical RAM, although a single process can only see 4GB at a time in its virtual address space. A CPU of this sort can be fitted with more than 4GB of RAM and can support multiple processes using more than 4GB of RAM in total. However, a single process can only use 4GB of RAM (minus some overhead from the operating system) at any given time.
On the Xeon this facility is known as Physical Address Extension (PAE) and some operating systems have APIs that allow a process to manipulate the MMU to swap physical memory in and out of the virtual address space of the process. On Windows this API is known as Address Windowing Extensions (AWE) but it is not available on all versions of Windows. Linux and other species of Unix support a similar mechanism.
In order to use this facility the program must be explicitly designed to support it - AWE does not automatically expand the virtual address space of the process. For example, certain versions of SQL Server support it, although I'm not aware of IIS doing so.
EDIT: Here we go. Found it. This posting on Stackoverflow links out to more detail on the mechanics of PAE.
Edit 2: If this forum posting is to be believed then IIS has no support for AWE.