Key TakeawaysWPP saw sales in North America and China fall, sending fourth-quarter revenue lower.CEO Mark Read said sales were "impacted by weaker client discretionary spend."The big London-based advertising firm also gave full-year guidance that missed estimates. U.S.-listed shares of WPP Plc (WPP) sank 15% Thursday when the giant advertising firm's revenue declined and it gave soft guidance as North America and China sales slid. The London-based owner of agencies including GroupM and VML reported fourth-q ...