True, Snuffy. I believe there were 5 Beatles at one time - Lennon, McCartney, Harrison, Best and Sutcliffe. Sutcliffe departed, leaving the band as a quartet, which they remained for the rest of their career as a band. Sutcliffe was usually referred to as the "5th Beatle" in the days before Pete Best was replaced by Ringo Starr (which happened in 1962 at about the time they signed for EMI).

The "5th Beatle" tag sometimes applied to George Martin has only been done retrospectively AFAIK - slightly confusing for those who remember Stuart Sutcliffe being referred to thus, e.g. if you had a pub quiz question "Who was known as the 5th Beatle?" many would answer George Martin without being aware of Sutcliffe.