I'll give "Who are You" my vote for 3rd place that season (Part One of that tale was just set up, all the fun stuff happened in Part Two). And Hush is a solid 2nd place - great villains and a neat gimmic. But my season four winner.....
Wild at Heart
This was the penultimate Willow episode of the first four seasons. The exchanges between her and Oz in that episode are terrific. It is also the first time that she's really has to face major negative consequences of her use of magic. And even though we did see Oz a few times after this, it was for all intensive purposes Seth Green's swan song from the series. I would suggest that any of you with the DVD's should rewatch this episode again before you cast your vote.