Well it would seem that there aren't too many of us with the Matrox system.

However my problem of the lost 4th screen is resolved.

For a completely different reason I regenerated the fsx.cfg.  Deleted it, ran the game, and lo and behold my 4th screen went dark, and I could transfer pop-ups across to it.

How was it resolved, why, I simply don't know.  As I do not believe in parallel spiritual worlds, there must be an explanation, but with so much to do with FSX, it is probably so arcane as to be incomprehensible.