Sarah is set to work along with a previous foe while another Leda clone is set to be introduced in "Orphan Black" season 4 episode 4.

"Orphan Black" season 4 episode 4 is entitled "From Instinct to Rational Control." In a promo trailer for the series' fourth episode, it is seen that Sarah interrogated Ferdinand on what the latter desires. As per the clip, the latter wants the former to let him know her source for both of them to be knowledgeable on what they really need.

The trailer also sees Sarah stunned by a revelation concerning the newest clone called MK. According to Design and Trend, MK is Sarah's sister and the latter would find out that MK is highly connected to a similarly inexplicable clone whom she may have been working along with.

The aforementioned events in the trailer suggest that Sarah has realized that he former enemy is aiming similar goals, just like hers; thus, she opted to form a risky alliance with the latter for her to be able to go to Neolution's leader.

Aside from some Sarah-centric events, "Orphan Black" season 4 episode 4 is also set to see Alison give an assignment to Donnie and Felix to penetrate a Neolution fertility clinic that has been scrutinized by Beth before. In fact, the duo have to pretend as a gay couple to do the job properly, according to Ecumenical News. Whether they are set to succeed in their task is yet to be revealed.

Also, Scott and Cosima are set to get a bad package from Alison while Susan tells Rachel something devastating, not to mention that the latter is becoming closer to Charlotte.

"Orphan Black" season 4 episode, "From Instinct to Rational Control," is set to air on May 5. Watch the promo trailer here.