You don't need one monitor per GPU. Also he wouldn't get a single virtual display that way only separate screens. For triple-monitor gaming for example you'd have all three on the same card and if you added another card for SLI you wouldn't attach any monitors to it.
4xSLI also causes a world of problems so I'm not quite sure what he's doing there.
*The GTX970 and modern AMD cards often support THREE monitors on a single card in individual or surround (virtual). You must have monitors that support the proper inputs such as DisplayPort, DVI depending on the card used.
For example, some support 3xDP (one per monitor) with HDMI or DVI capable of driving an individual fourth monitor (for triple-monitor gaming plus another screen.)
**NVidia has a section for SLI that explains this in more detail based on the cards chosen.