Sonya infiltrates Shefner’s apartment looking for a connection to the victim. She discovers missing military books from Volynin's home and a note in Shefner’s diary confirming a meeting with him on the day he died.
Just as Petr Natanovich joins the investigation, Volynin's body disappears from the morgue. While this causes panic for Korzun, Katerina views it as proof that the death was a calculated murder rather than an accident.
This season focuses on how the women adapt following the departure of Lieutenant Colonel Proskuira (played by the late Sergei Puskepalis).
She struggles with a secret pregnancy by Letnev, which she hides from her peers.
The tension peaks when Shefner catches Sonya searching through his personal belongings. Character Arcs in Season 4
The season follows the series' established structure, where every four episodes typically cover one major criminal case. Шифр (телесериал) - Википедия