With time running out, Retsuko and her friends (and Ton) must gather evidence of fraud of the company before the ledger numbers are submitted.
After confronting both Retsuko and Tadano, Haida must determine what's truly important, even if he risks not only his new promotion, but his job.
Tropes:
- Back for the Finale: The OTM Girls' manager Hyodo returns from season 3. He discovers Retsuko's popularity on her channel and, after running into her again, attemps to invite her for a collaboration to help his idols. While she shoots down the idea, she does call on him to help investigate the ledger fraud.
- Bait-and-Switch: Haida fully expected Retsuko to do a metal song during a confrontation. Instead, she slaps him to make him come to his senses.
- Bittersweet Ending: With everyone's help, Himuro is forced to stop his plan to submit the fraudulent ledger, saving the company from possible legal backlash. With him stepping down, Shachou returns as CEO following his recovery. Although Ton and Kabae are rehired, Haida decides to resign himself following his actions. Despite this, Retsuko and Haida continue to hang out.
- Destination Defenestration: Retsuko somehow blasts Haida and Himuro through an office window with her metal voice. Luckily, a scaffold catches their fall.
- Exact Words: Washimi previously told Himuro that Retsuko is known to be "obedient" . After the incident with her and Haida, he learns that Washimi meant Retsuko is "obedient to her own heart".
- Gale-Force Sound: Tired of the stalemate between Haida and Himuro, she uses her metal voice to break the fight, strong enough to literally blast them out of the office window!
- Ironic Echo: Both Tadano and Himuro tell Haida that "he's picking a fight with the wrong guy". Haida realizes that he should talk with Retsuko instead of picking a fight with Tadano, but believes he's right in standing up to Himuro.
- Maybe Ever After: Haida and Retsuko meet up to walk together and look very happy with Haida feeling he is worthy of Retsuko. It is still ambiguous whether said outing was an actual date.
- One-Sided Arm-Wrestling: Haida challenges both Tadano and Himuro to an arm wrestling match. It seems as though Tadano is on the losing end, but manages to fight back and beat Haida. The match with Himuro is even less one-sided because both he and Haida have been working out, making this match nearly even.
- Shut Up, Hannibal!: Haida tries to reason that he's knowingly performing fraud to improve his future with Retsuko, but she tells him that it's not a valid excuse since he chose this path, plus she never wanted him to take those measures.
- Status Quo Is God: After Himuro steps down from the title of CEO, Shachou's yes men believe they finally have a shot at taking over... Until Shachou announces that he's coming back after a full recovery. Ton and Kabae are returning to the company as well. The only exception to this return to normalcy is Haida leaving.
- The Stinger:
- A short scene plays in the middle of the credits: Tadano is still sore after his arm wrestling match with Haida. His AI companion, ENI-O, criticizes him for overdoing it just to prove a point.
- After the credits, a short message states that "RETSUKO WILL RETURN" (with "5" briefly appearing in the background, effectively confirming a fifth season).
- Whatever Happened to the Mouse?: Are the other company workers, including Inui, back as well after their resignation?
- "Will Return" Caption: After the credits, the message "RETSUKO WILL RETURN" appears. A number 5 briefly appears in the background as well, all but stating that a fifth season is on the way.
- You Are Better Than You Think You Are: Retsuko asks Haida if Himuro made him commit ledger fraud, and Haida respond as if she's insulting his competence and ambition. Retsuko slaps Haida and says she thought he was too good to commit that kinda crime, begging him to submit the real books and return to the person he once was.
