The challenge for Antony Hegarty is just how best to use that quivering, purring, sobbing, ecstatic, altogether original voice. Though he’s a great disco diva (see Hercules and Love Affair’s 2008 “Blind”), his Johnsons work tends toward the sepulchral. On their fourth set, ghosts still haunt Hegarty’s piano-and-string-sketched chamber songs. But there’s a new ease to his musical language here, and playfulness, too. He sings a breathtaking duet with Björk in Icelandic (“Flétta”). And the left hook is “Thank You for Your Love,” which fully realizes the Southern soul sound Hegarty has flirted with on older tracks like “Aeon” and “Fistful of Love.” The further afield dude wanders, the better he gets.