Song titles on John Mayer's fourth album remind us that love is a battlefield: "Heartbreak Warfare," "War of My Life," "Assassin." Broken hearts figure in the set's first two songs; by the third, a duet with Taylor Swift, Mayer's heart has split in half. Sometimes his response to emotional combat is to be a conscientious objector, writing declarations of independence. But in the end, he's just a vulnerable guy. So his guitar solos mostly stay sensitive; occasionally, though, he beefs things up a bit. And when he revives Robert Johnson's "Crossroads," he gives it some real Stevie Wonder funk.