Who directed parts three and four of the finale Sozin's Comet?

Joaquim Dos Santos directed parts three and four of the finale Sozin's Comet.

The finale uses three directors across its four chapters. Ethan Spaulding directed part one, “The Phoenix King,” Giancarlo Volpe directed part two, “The Old Masters,” and Joaquim Dos Santos directed parts three and four, “Into the Inferno” and “Avatar Aang.” This division is why the answer is a single director even though the overall finale has multiple directing credits.

Dos Santos was already an important Avatar contributor before the finale. He directed several episodes and helped shape the show's kinetic action style, which is especially visible in the final two chapters. Those episodes contain Aang's decisive fight with Fire Lord Ozai, Zuko and Katara's Agni Kai-related confrontation with Azula, and the White Lotus liberation of Ba Sing Se.

Dos Santos's work on part three also received major recognition: he won an Annie Award for directing in an animated television production. A common mix-up is naming Giancarlo Volpe, who directed part two, or Ethan Spaulding, who directed the opening chapter. Neither directed the finale's final two parts.