I think there's more to S4 than the standalones. The Big Bad that season is sort of a let-down but everything that happens in S4 foreshadows what's to come for the remainder of the show. the "standalones" really aren't so much - WAY, for instance, foreshadows Buffy and Spike in a way that's darker and more real to what happens in S6 than SB (although the bathtub scene in SB is aces.) It's the first time Tara "sees" the real Buffy in a way that her friends don't, etc