Ok, ran a few more tests just for the heck of it (and in case it helps others) since, as mentioned above, I don't plan to use V4 further so this is of limited practical value to me personally. 

Results of first tests implied that the only thing that mattered for causing the flying jaw was whether the V4 eyes were set to PointAt a camera or an object parented to a camera (any camera, not just the one used for the current display).

When I reparented the PointAt target prop to UNIVERSE, the jaw behaved properly again.  But then the flying jaw returned if I subsequently reparented to ANY prop or figure below the UNIVERSE level.

But then I followed ThrommArcadia's suggested experiment (at least as I understood it).  I applied a new V4 pose in the last frame and got the flying jaw, but switched to the Face Cam and rotated it to snap the jaw back in, then hit Undo.  Very surprisingly, the jaw then worked properly throughout the animation, even when the PointAt target prop was reparented to a camera.  That is, I could then actually run an animation with the eyes pointed at a camera and the jaw staying in place.  However, I tried to repeat the procedure and it didn't work the second time. 

Perhaps others could keep experimenting with this, though, since it's past my bedtime now and, as already stated, my motivation for exploring V4's issues is pretty limited.

In any case, thanks to ThrommArcadia and others for the thoughtful ideas and suggestions.

cheers...