My choice for Fifth Beatle would have to be George Martin, the Beatles producer. Martin captured their early brilliance and assisted in their musical education by providing access to instruments, electronics and the capabilities of the recording studio to help them realize their sound. John Lennon often said that Martin's role was overplayed, but the fact of the matter is that Martin along with engineer Geoff Emerick, and a handful of others, were the only other people in the studio helping the Beatles to create the sounds they imagined, described and demanded.
It was Martin who suggested speeding up the tempo on the Beatles first number one single, "Please Please Me". It was Martin's classical training and impeccable orchestral arrangements that helped make "Yesterday" , "Eleanor Rigby" and "Something" stand far above the cheesy pop ballads of the time. He encouraged the Beatle to explore new sounds and adapted his working methods to compliment their unique styles. When John came to him with two versions of "Strawberry Fields" Martin worked out a way to bring the two together. When Paul sang the piccolo trumpet line that he wanted in "Penny Lane", Martin transcribed the tune so it could be read by a professional player. Martin always speaks of how Paul tended to be very succinct about what he was looking for and that John would be more descriptive of the mood, emotion or feel of the song. How Paul was looking for a very clean sound, whereas Lennon wanted a dirtier sound. Martin helped deliver that sound, helped make it cohesive album-to-album even as recording standards changed throughout the decade. He taught them how to use the studio as an instrument in it's own right. And, as he has always said, the more he showed them the more they wanted to learn. When Lennon complained about having to double track vocals during the making of "Revolver", a signature part of the Beatle sound, Ken Townsend, one of Martin's engineers developed Artificial Double Tracking (ATD) eliminating the need and time required to duplicate the original vocal. Sound sampling, close micing of acoustic instruments, backwards tape, even the first instance of recorded feedback on 1965"s "I Fell Fine" were enabled by this extraordinarily resourceful and creative producer. He and the Beatles were an ideal match.
Martin shared the Beatles sense of humor from the start and quickly saw their musical potential. He expressed regret in not encouraging George Harrison more than he did, but he always insisted that there was something very unique at work when all four Beatles were in the studio.