Rebecca Mader will guest in the Season 4 finale. Mild spoilers follow..
The LOST alum will appear in the 2-part finale as Jessica — a brave, strong woman with a link to David Robert Jones.
Jessica will visit Olivia and go on to interact with both Peter and Walter.
J.H. Wyman notes:
“She will have an emotional connection with Olivia. They may find out they have more in common then they first thought.”
As for whether Jessica is a love interest for Peter or if she could return in the potential fifth season, Jeff Pinkner adds:
“To answer those questions would reveal a little too much. The finale in many ways is our characters’ final battle.”
Source: TV Guide