In Niles and Chamberlain’s “Dead Billy Dead, Part 4,” Billy’s relationship, rocky even before he became one of the undead, takes a definite turn for the worse. And in part 4 of “Juarez, or Lex Nova and the Case of the 400 Dead Mexican Girls,” Fraction and Templesmith present a freshly dug grave, an exhumed body, more dead goats, a party, a hunt, an intervention, and some actual detective work from Lex.