Okay, yes, Michael is most certainly dead as the last scene of GF3 is entitled "The Death of Michael Corleone." The writing was bad, the sepia tones were gone, the movie was too modern, and Sofia Copoola was crappy, especially at the end. But if a GF4 would take place, it would likely be between 1927 and 1945 (the mobb war of the depression), 1950 and 1958 (the rise of power and moral downfall of Michael), 1959 and 1978 (Michael's drive to be legitimate), or Vicent as a Don.