A good GF IV could be made using a similar structure to GF II (i.e. flashbacks and 'modern times).

The 'flashback sequences would center around the 'happy years' of 1927-1941 when the Corleone Family rose from essentially a neighborhood operation to a dominant force on the national crime scene.

The 'modern' sequence would focus how the family functions after the hit attempt on Michael and Mary's death, and what effect this has on the survivors. The time span would be mainly in the 1980s-90s since the time of Michael's death as seen in GF III is said to be approximately 1997.

The story would depend on Al Pacino delivering an absolutely nihilistic performance as a declining, embittered Michael, and also on Talia Shire as a chillingly ruthless and relentless Connie, who I could see in GF IV perhaps becoming the final embodiment of the family's legacy of death.