4x16 - The O.C.

The Cohen family says goodbye in the series finale as the episode jumps six months after the earthquake to show the Cohens, Ryan, Taylor, Summer and the Cooper women trying to piece their lives back together. The Cohens find out that fixing up their old mansion would cost more than rebuilding it but Ryan and Seth see this as an opportunity to give Sandy and an expecting Kirsten a surprise that'll change their lives.