The answer ... technically ... is 5.

The band formed around 1981 with Ron McGovney holding down the low end.

He was replaced pretty quickly by the phenomenal Cliff Burton, (Ex - Trauma), who would write and record with Metallica through their first 3 albums; Kill 'Em All, Ride The Lightning and Master Of Puppets. Sadly, just as the band really began to peak, he was killed in a bus accident in Sweden.

Next up came Jason Newsted. The former Flotsam And Jetsam bassist came into the fold with a new angle on the rhythm section and helped propel them to even greater heights with records like ... And Justice For All, Metallica, Load, Reload, Garage Inc and the bands pairing with the San Fransisco Philharmonic, S & M.

Here's the "technical" one I alluded to in the opening sentence.  While working on the St. Anger record, producer Bob Rock sat in on bass. He also did a couple of live performances with the band.

During the recording of St. Anger, Metallica interviewed just about every bassist out there. (Guys like Pepper Keenan, Jeordie "Twiggy Ramirez" White, Scott Reeder and Les Claypool just to name a few.)

After all the auditions ... and a complete band breakdown ... Suicidal Tendencies, Infectious Grooves and Ozzy bassist Robert Trujillo landed the gig. He's since "crab-walked" his way through Death Magnetic, Lulu, and Hardwired ... To Self Destruct.

This is where I could say "and the rest is history" but, it's not.  It's a story that continues to be written. Be a part of the next chapter February 28th, 2019 when Metallica FINALLY return to the Sun City.