Season 4 finale, The Deconstruction of Falling Stars. It wasn't originally planned, they needed a filler episode when they got renewed. Basically when they rea

Season 4 finale, The Deconstruction of Falling Stars. It wasn't originally planned, they needed a filler episode when they got renewed. Basically when they reached season 4, they got worried they wouldn't get renewed for the planned 5th season so secondary plots for both seasons were dropped and the primary plots for both were compressed into season 4, including the series finale. Then they got renewed for the 5th season and couldn't air that finale until the actual end of the series, so they used some budget from the 5th season to create the new 4th season finale. This is also why Ivanova is in the series finale even though the actress had left the show, it's the one that was filmed the previous year while she was still there.