Metallica are teasing a big announcement regarding 'The Big 4' on their website and Facebook page. The Big 4 was the name of the concert featuring Metallica, Slayer, Megadeth and Anthrax on 22 June 2010 at the Sonisphere Festival at Vasil Levski National Stadium, Sofia, Bulgaria. Check out this video to get some idea of how manic the concert was: