With some make up done to make him look younger, and some hair dye, could De Niro still convincingly play a middle aged Vito (IE a Vito somewhere between 1925 and 1945)?

I think instead of a "Godfather IV" focusing on the decline and fall of the Family from the '80s to the '90s under Vincent, they should do a film which focuses on the rest of Vito's rise to being a kingpin between 1925 and 1945--The organizing of the Five Families and Commission by Vito, the recruitment of Luca, Sonny's entrance into the Family, the first war...Maybe stopping at when Vito buys the house in Long Beach in 1939. Sort of like how GF I ends as Michael is selling the house in Long Beach in 1955.

Or do what GF II did--show Vito's rise to greatness and power from 1925-1939, contrasting it with the final decline and end of the Corleone Family under Vincent in the '80s-'90s.

Like have it pick up maybe a few years after GF III--Maybe 1983--and have it end in 1993 in an ending similar to the fate Escobar met: Shot to death with a crumbled empire. Or base Vincent's end on John Gotti's or something, basically Vincent leading the Family to degradation to a return to street-like rule and drugs...Basically, being what his father would've been in peace time.

Meanwhile, you'd see Vito rising to the heights of power honorably.