Task: Question Answering
Input (English): Passage: Metallica is an American heavy metal band. The band was formed in 1981 in Los Angeles, California by drummer Lars Ulrich and vocalist/guitarist James Hetfield, and has been based in San Francisco, California for most of its career. The group's fast tempos, instrumentals and aggressive musicianship made them one of the founding "big four" bands of thrash metal, alongside Megadeth, Anthrax and Slayer. Metallica's current lineup comprises founding members Hetfield and Ulrich, longtime lead guitarist Kirk Hammett and bassist Robert Trujillo. Guitarist Dave Mustaine (who went on to form Megadeth) and bassists Ron McGovney, Cliff Burton and Jason Newsted are former members of the band.
Metallica earned a growing fan base in the underground music community and won critical acclaim with its first five albums. The band's third album, Master of Puppets (1986), was described as one of the heaviest and most influential thrash metal albums; its eponymous fifth album, Metallica (1991), the band's first to root predominantly in heavy metal, appealed to a more mainstream audience, achieving substantial commercial success and selling over 16 million copies in the United States to date, making it the best-selling album of the SoundScan era. After experimenting with different genres and directions in subsequent releases, the band returned to its thrash metal roots with the release of its ninth album, Death Magnetic (2008), which drew similar praise to that of the band's earlier albums.
In 2000, Metallica led the case against the peer-to-peer file sharing service Napster, in which the band and several other artists filed lawsuits against the service for sharing their copyright-protected material without consent; after reaching a settlement, Napster became a pay-to-use service in 2003. Metallica was the subject of the acclaimed 2004 documentary film Some Kind of Monster, which documented the troubled production of the band's eighth album, St. Anger (2003), and the internal struggles within the band at the time. In 2009, Metallica was inducted into the Rock and Roll Hall of Fame. The band wrote the screenplay for and starred in the 2013 IMAX concert film Metallica: Through the Never, in which the band performed live against a fictional thriller storyline. 
Question: What were the founding "big four" bands of thrash metal?
Output (English): Megadeth.