Answer:
Day & Night, Yin/Yang
Step-by-step explanation:
My friend and musician told me once that he considered Paul McCartney & Ringo Star as the 'Daylight' musicians and John Lennon & George Harrison as the 'Night' musicians. He 'proved' this by making a daylight compilation with songs of Paul McCartney and a night compilation with songs of John Lennon and George Harrison. Whether or not his theory is true, it doesn´t matter. What does, is the fact that there was a unique balance of four characters. Two of them (daylight) realistic and two of them (night) idealistic.
Brian Epstein, called the 5th Beatle, was the manager and very important for selling the product called the Beatles.
George Martin, likewise called the 5th Beatle, was tremendously important as the (artistic) producer of the Beatles. Just listen to the early Beatles records (very simple music, technically speaking) and compare with later work like the song I want you (the changes in rhythym for example). John Lennon and especially Paul McCartney clearly learned a lot of Sir Martin.