This episode's headline: Securing the Fourth; Music Volt; Level Playing Fields; Conversation; Sparklers. ANCHOR: JIM LEHRER; GUESTS: TERESA CHAMBERS; DONNA LOPIANO; MICHAEL MOYER; MARCIA GREENBERGER; MELANA ZYLA VICKERS; DAVID McCULLOUGH;ROBERT PINSKY; CORRESPONDENTS: KWAME HOLMAN; RAY SUAREZ; SPENCER MICHELS; MARGARET WARNER; GWEN IFILL; TERENCE SMITH; KWAME HOLMAN