In the coda, over the repeated line "Four Seasons, you'd better believe it," Brian sings a wordless falsetto part, something like this:
This is taken almost directly from the Four Seasons' "Walk Like a Man." At the beginning, Frankie Valli sings his own falsetto part, something like this:
(With some small variation, these phrases also appear in the middle of the song and again in the coda.)
There's a slight difference in the rhythm in the second measure, but the intervals are exactly the same (in the top line, at least).
