Our season-by-season survey of “Buffy the Vampire Slayer” goes to college, as we explore season four. UC Sunnydale is full of welcoming professors, friendly TAs, annoying roommates, and a collection of government commandos who are experimenting on the local demons and vampires. Spike returns as comic relief. We’re all about Willow and Tara. And then there’s that episode where nobody talks and that other one where they’re all dreaming. We break it all down like it’s 1999.