Complete story — includes the ending
Peter Bishop has returned to the altered timeline, but no one recognizes him. Walter refuses to have anything to do with him. Meanwhile, the Fringe division deals with the threat of the translucent shapeshifters when a former Massive Dynamic scientist is taken by a shapeshifter, who forces him to create a serum that will enable them to change their form more quickly. Eventually, the shapeshifter escapes and the Fringe team is no closer than before.
Sources and content license
Source and revision history · Adapted from Wikipedia contributors, Fringe season 4. HTML formatting and reference markers removed. · Content license