Worf is caught in the middle of a Klingon civil war.
Picard takes the Enterprise to the Klingon homeworld to complete his role as Arbiter of Succession and install Gowron as leader of the High Council. Worf also hopes to restore his family's honour. Lursa and B'Etor, the sisters of Duras, put forward Duras's son Toral as a rival claimant.
Picard rules that Toral's claim is not valid, and civil war breaks out between Gowron's forces and the Duras family. The Duras sisters have secret help from the Romulans, but the Federation will not take sides. Worf resigns his Starfleet commission and joins his brother Kurn in fighting for Gowron, who restores the honour of their family.
The fourth season ends on a cliffhanger with the reveal of Sela, a Romulan commander who looks just like Tasha Yar.