I dunno if cycle length is the key here, the Soviets and the Chinese went with five-year plans, and done properly, it seems like thats a long enough amount of time to accomplish very important things.

WW2 took slightly less than 6 years, when we count it from the invasion of Poland to the fall of Nazi Germany.

The moon landings took little less than 7 years, so I don't think we are terribly off by the timeframe.

Considering the world's been getting faster (just think about how different the US was before Trump took power a bit more than a year ago), I think 4 years is fine.