Back in the late 80s, computers had relatively little Random Access Memory; in fact they had too little to complete certain operations. Someone, and I believe it was Borland Software, worked out a system by which the hard disk could act as Random Access Memory. It was definately not faster, as hard drives were, and still are slower than RAM. Nevertheless, memory intensive functions could at least be performed. This was called Virtual Memory, because it wasn't really RAM, but it looked and acted like it was.
Interestingly, at around the same time, there were hard disk functions that were simply too slow. Program and RAM permitting, they could be loaded into Volitile Memory (RAM) and worked much faster. The space allocated for this function was quite small, but sometimes very useful. This is what is meant by Virtual Disk, and I'm not sure it is even used today.
That 4GB RAM is not Virtual Memory. It is the real thing. If you are not playing games, 4gig should be adequate, even for VISTA. Windows 7 may be less of a memory hog than VISTA.
I skimmed the wilki article. Offhand, my impression was that it was sort of a Virtual Explanation. It looked and acted like a real explanation, but I too would have to spend a bit more time than I want to see if it were the real thing.