There goes FFC contributing to the confusion:

Immediately after Sonny died in 1946 (which I believe), Vito, as we saw, told Tom, "This war ends now," and to arrange a meeting with the heads of the Families. The novel says it took a year after Vito arranged the peace at that meeting for Michael to have returned to America--putting his return in 1947, not 1950. And, if it were 1950, Vito--sick, dispirited, trying to look weak--would have had to run the family by himself for four years, if there was anything left of his family, since the others would have taken advantage of his weakness and taken everything.