I'm not so sure about those semantics. Even so, I can't see it taking four years to execute the plan.
Why not?
Vito was a deliberate man. He originally gave his word (to Barzini and the others) that he wouldn't be the one to break the peace. That bought him time to school Mike in the Family matters and time to lull the other Families into a sense of calm.
It makes perfect sense, IMO, that Mike and Vito DEVISED the plan to kill the heads of the Five Families in 1950 (shortly after Mike returned from Sicily) and act upon it (much) later when they were fully prepared to do so.
