I’m notorious at forgetting shortcuts in Blender: just as I had remembered that any viewport can be switched into a quad view with CTRL+ALT+Q, I was stuck with another problem: the views I was seeing were locked, and I couldn’t change say a back view to a front view, or a top view into a bottom view.
Of course there’s a (hidden) way to change this by use of the ominous Properties Panel. Open it with N, then navigate to View – Quad View and disable Lock Rotation.
Now you can hover over any of the four view ports and use the usual shortcut keys like 5 to toggle perspective, or CTRL Numpad combos to look at the opposite side of your model.
Quad view is different from creating four different viewports in that all viewports share the same render properties, and it provides a quick way to see your model from all sides with a single shortcut. So yes, the Blender take-away for today is: CTRL+ALT+Q, followed by N.