I think it is not realistic that somebody, even though the son of a Mafia boss, rises to the top of boss in only a few years after becoming involved. I think eight or nine years being groomed for the top spot is perfectly normal.
Was Vito lamenting that 4 or 5 years wasn't enough time, or that 8 or 9 years wasn't enough time?
Therein probably lies the answer. But there's no answer beyond anyone's own conjecture. The timeline was the weakest part of both the book and the films.
But when they wrote this stuff over forty years ago, they weren't picturing a bunch of lunatics with no lives (like us) discussing it on the Internet all day (Or as FFC famously asked Puzo during filming in 1971: What's the Internet?
).
