I'm working in a file that has been converted/imported from Maya (not by me). This file seems to be somehow locked in single-viewport display mode. Even if I go into customize viewport and select a four-view layout, when I OK it, I see the four frames rendering briefly, and then it shows only one of the viewports across the entire screen again. Arrgh! What's wrong? How do I fix it?

Solved my problem.

I'm using oFusion Community Edition (for Ogre export), and apparently it has a zoom function. When zoomed, it will make any viewport be zoomed. Switching to an oFusion viewport, I could then un-zoom it, and all is well.