As previously reported here, Lars Ulrich did attend American Carnage show at Cow Palace in San Francisco. A Megadeth Fan with handle TheSorrow posted a video of Megadeh ripping into “My Last Words” after Dave Mustaine mentioned The Big Four US Tour and Lars Ulrich in the middle of “Peace Sells.” Mustaine even mentioned that Lars was backstage requesting Megadeth to play the song.
Listen to Dave Mustaine starting at 3 minute mark.
Based on my sources they want The Big Four US Tour to happen. Metallica, Slayer, Megadeth and Anthrax are still working on the details.