clearly stated: the Byrds quote
obvious: the 'Pure and Easy' quote from Pete Townsend (extra credit for all three times it was used on LP's)
Less obvious (but you guys already got it): the Zabriskie Point reference, with all the cool noodlings.
The fourth one is the one you'll never get, and that's for a number of reasons. Easy when you start from the end, but not from the other direction.