^^The above explanation makes no sense.
BTW, the linux kernel has a 3/1 split. The split refers to the virtual address, not physical.
Don't confuse physical and virtual memory.
32 bit computers used to have only a 32 bit physical address space. Which would theoretically allow for 4GB of physical memory. Practically however, a lot of that space is used by devices (memory mapped I/O). Later, x86 procs had PAE added to them. This allowed a 36 bit address space which allowed for more than 4GB of physical address space. This means that you can address 4 GB of physical memory (and more) plus the memory mapped I/O too.
Only issue? A lot of 32 bit drivers wouldn't be able to handle the extra address space (even though some would appear to do so, they would silently fail). So Microsoft decided that if you wanted the extra memory, you could use a 64 bit OS. Hence they only reported the amount of memory they mapped into the physical address space (which was less than 4GB). However this was a bad UI decision - everyone looks at it and starts freaking out ("oh no! One of my DIMMs has gone bad! Must contact Dell NOW!"). So in Vista SP1 they fixed it to show the memory present in the system.