At the beginning of the party scene, when Jin serves Sun a glass of champagne, the classical music playing in the background incorporates both the numbers 42 and especially 4. It is the opening of the second movement, the minuet, of the String Quartet in D minor, opus 42, by Joseph Haydn (1732-1809).
Haydn is credited with creating the string quartet as a distinct musical form, consisting of an ensemble of 4 instruments--a pair of violins, a viola, and a cello. Most string quartets, like this particular one, have 4 movements. This work's overall key, D minor, also includes the fourth letter of the alphabet. A considerably longer selection from the second movement of this same Haydn string quartet is heard again playing faintly in the background in the Season 2 episode …And Found, when Sun first meets her prospective suitor at the hotel.