Mike-
With the current (2000-present), a GF IV movie would NOT fail at the box office. Most likely it would open rather largely at number one, with i would say about 25-30 million. We've had the discussion before about how time has healed Godfather Part III. i know, you've said repeatedly, that you are disappointed in III, but the casual fan anymore seems to ignore any inferiority.
A movie like this (a fourth entry in a series) with a decent cast (bringing back Garcia, Shire, and Keaton or even Fonda) would top out between 90 -100 mill. hardly a roaring success, but not a failure.
This world we live in is different from 1990. Sequels of this magnitude are much more widely accepted and less scrutinized.
Though the fact remains, a Part IV makes no sense, it is money, no doubt.