The Mezzanine

I saw someone a while ago point out that season 4 is rather essential – in that, for any other season, you could skip it and go straight from the season before to the season after, and you'd still be pretty up with events. Whereas, if you skip season 4? Willow is gay, Spike is a Scooby, and pretty much every character is doing markedly different things to what they were doing in season 3.

Everything you said about S4 as the transitional season is spot-on; or at least that's how I always think of it, "the bridge season" between the two mini arcs of 1-3 and 5-7. It is essential, and if you weren't paying attention you'll miss a lot. I was surprised when I rewatched the very first ep, the Freshman, that Giles pushes Buffy away (I'm not your Watcher anymore); of course he comes running back but it started there, it didn't just happen out of the blue.

And WAY (I just left a comment on your day 2 post) foreshadows pretty much the entirety of Buffy's S6 arc, and the direction Spuffy will take. It's a funny season but I love the dark undercurrents still running beneath it.

And - TARA! CAVE BUFFY! What is not to love?