Severian reaches the war, finds the Autarch, and learns the truth about his world, his mission, and himself. Wolfe concludes The Book of the New Sun with revelations that recontextualize every page of the preceding three books. The most rewarding series finale in literary fantasy — if you can decode it.
Skip if you dislike:
Moods: War Revelatory Final Transcendent
Tropes: War Autarch Revealed New Sun Identity Transformation
It answers almost everything — if you can read between Wolfe's lines.
The Urth of the New Sun is a coda — optional but rewarding.
Of the main tetralogy, yes. Wolfe's greatest achievement is complete.