For that matter, the only indication that the death of Michael Corleone took place in 1997 is off-camera commentary by FFC.

There is nothing in-story continuity in GF III that indicates any particular year it has to be.

Obviously Michael is very old and now mostly blind, but really that could be anywhere from 1985 to 2015.

Michael was born in 1920, so he could be an aged and sickly 65, or he might be a well-preserved 95. Some people do reach a kind of 'aging plateau' and don't look much different at 90 than they did at 70.



So that could open quite a bit of flexibility for a possible GF IV. The date of the death scene isn't set in stone.