Sarah creates an alliance with a former nemesis to infiltrate Neolution in the upcoming episode 4, titled "From Instinct to Rational Control," of the fourth season of the science fiction drama "Orphan Black."

Sarah now understands that her former nemesis has the same goals as her, so she decides to form a dangerous alliance so she could go to the head of Neolution. Alison, on the other hand, gives a task to Felix and Donnie to infiltrate a Neolution fertility clinic that Beth once investigated. Meanwhile, Cosima and Scott receive an unpleasant package from Alison. Susan Duncan gives a devastating decision to Rachel who is growing closer to Charlotte.

The promo trailer for "From Instinct to Rational Control" shows Sarah asking Ferdinand what he wants. He wants her to share her source so they both know what they need. Sarah discovers some shocking information about the newest revealed clone, MK. Meanwhile, Helena consoles Donnie as he was tasked by Alison, together with Felix, to pose as a gay couple to infiltrate the Neolution fertility clinic. Lastly, MK does her own investigation about another clone.

In other related news, creators of the show shared about Delphine's disappearance in an interview with Entertainment Weekly. Graeme Manson said, "Cosima's struggling with that disappearance. We're going to keep struggling with that with this season. It's part of the mystery, finding that answer. And we promise tears and heartbreak along the way."

John Fawcett also shared a bit of the upcoming episode 4. He said that it will be a "real re-dosing of MK" and that includes some of her darkest and deepest secrets.

The previous episode, titled "The Stigmata of Progress," saw the shocking discovery that Sarah found out. Her desperation to find answers pushed her to follow a lead that only got her into a harrowing encounter.

"Orphan Black" season 4 airs every Thursday at 10 p.m. on BBC America.