Last night, the so called "Big Four" of the '80s thrash-metal boom -- that'd be Metallica, Slayer, Megadeth, and Anthrax -- came together at Yankee Stadium to play only their second-ever U.S. show together, following the one they played in California a few months back. And during Metallica's encore, members of all four bands, as well as '80s peers Exodus, ...