Over the past three months, four superstars have released albums that assess, from different angles, what a genre-dominating rapper does when the genre is beginning to move on: Kanye West’s “Ye” (as well as his collaborations with others); Drake’s “Scorpion”; J. Cole’s “KOD”; and Jay-Z’s “Everything Is Love,” which he and his wife, Beyoncé, put out as the Carters.