Metal's Big Four are coming off their first-ever show together in the U.S.  The bands played at the Empire Polo Club in California.  Just one week earlier, the same venue had hosted this year's Coachella festival.Attracting thousands to the California desert wasn't the only "win" enjoyed by metal's "Big Four" over the weekend.  Blabbermouth.net reports that the RIAA presented Metallica, Megadeth, Anthrax, and Slayer with double-platinum plaques on Saturday during their concert in Indio.  The plaques commemorate sales of the bands' two-disc DVD release "The Big Four: Live From Sofia, Bulgaria."  The Big Four produced the video with footage from their historic show across the Atlantic on June 22nd, 2010, which was beamed to movie theaters worldwide.  Sales of "Live from Sofia, Bulgaria" went double-platinum last December when unit sales exceeded the 100-thousand mark.