The fourth episode of "To My Beloved Thief" commences with a tense confrontation between Prince Yi-yeol and Jae-I at the Hyeminseo. Eun-jo, playing along with Jae-I's scheme, deceitfully informs Prince Yi-yeol that she loves him and will soon marry him. Her intoxicated brother, stumbling into the clinic, further solidifies the lie by referring to Jae-I as his in-law.
Eun-jo, consumed by her duties to her brother, leaves Prince Yi-yeol and Jae-I outside. Jae-I's belittling and insulting attitude towards Eun-jo infuriates Prince Yi-yeol, who is on the verge of striking him. As Eun-jo asks him to leave the Hyeminseo, her friends arrive with food and drinks, reminding her of the party she missed. Prince Yi-yeol is heartbroken, and his mood shifts dramatically over the next few days.
When the Queen Dowager learns of his condition, she sends a statement to the Hyeminseo, instructing all physicians to come together to find a cure for Prince Yi-yeol. Meanwhile, His Majesty suggests appointing Min-jik as the new Chief Censor, which angers Secretary Im and his son, Seung-jae. They even consider eliminating Min-jik, as his reinstatement would threaten their position.
Despite this, Secretary Im informs Min-jik of the appointment, but Min-jk declines. He explains that he does not want his family to relive the pain of the Gyeongju Year. At the same time, the Queen Dowager and the Queen Consort keep each other company, worried about Prince Yi-yeol's unpredictable behavior during his meeting with His Majesty and fearing a repeat of the Gyeongju Year incident.
Later, Min-jik picks up Eun-jo from work and they walk home together. He informs her that Secretary Im has recommended him for a teaching position and that he will not be around during her wedding. Eun-jo is concerned about his health since the academy is far away. However, his departure eases her tension about the impending marriage to the old and sickly man.
Meanwhile, Jae-I's quest to cancel his engagement to Hae-rim reveals that he is also a concubine's son. Secretary Im paid a hefty sum to silence his mother and raise him as the legal son of his wife because he only had one heir. Secretary Im finds out about his meeting with the Grand Prince at the Hyeminseo and panics, thinking that he meddled in his affairs, and slaps Jae-I.
Jae-I vents his anger by once again humiliating Eun-jo, who he encounters in the market, purchasing herbs for Min-jik. Her smile and concern for her family ignite his resentment. He desires to see her succumb to bitterness, just as he has. He forcibly drags her away, demanding that she cancels the engagement. She refuses, explaining that the marriage is a transaction for her, a means to save her family.
Meanwhile, Prince Yi-yeol persists in his pursuit despite Eun-jo's rejection. He attempts to sway her, urging her to choose him. Eun-jo insists on going through with the engagement, stating that, unlike Prince Yi-yeol, her social status does not afford her such luxuries. She dismisses his pursuit as a whim. He reminds her that it still takes courage to pursue the woman he loves, having had to carefully rehearse his words and approach her hesitantly. Prince Yi-yeol walks away, leaving Eun-jo heartbroken and confused.
Jae-I insists on calling off his engagement and this time threatens to reveal his true identity as a concubine's son to Hae-rim. Secretary Im advises him to accept his fate, just as Eun-jo did, and to keep the secret of his origin. Meanwhile, Dae-Il remembers Jae-I telling him that Eun-jo is to marry his grandfather and is extremely frustrated that Eun-jo chooses not to tell Min-jik the truth.
Elsewhere, Secretary Im meets with Chief Censor Kim in prison. Earlier, Chief Censor had discovered that Seung-jae was smuggling some seeds into the palace, which Geum-nok was burning as incense to drug His Majesty. He reveals this to Secretary Im, thinking it will be his bargaining chip.
Eun-jo's wedding day arrives, and it begins to rain heavily. Upon seeing her sister getting ready for the wedding, Dae-II reveals the truth to Chun-seom. Unfortunately, even Chun-seom's pleas cannot change Eun-jo's mind. On her way out to the palanquin, she finds that Prince Yi-yeol had gifted her silk shoes, which she tearfully leaves behind in a cabinet.
Meanwhile, deeply troubled by the impending engagement, Jae-I implores Hae-rim to call it off, fearing that they will both suffer together in the future. As he runs into Eun-jo outside their house, he angrily drags her into the rain-drenched night, growing increasingly frustrated with her steadfast determination to see the marriage through.
Just as Eun-jo steps through the threshold of her new home, she learns of her fiancé's untimely passing, making her a widow before she could even say "I do." Elsewhere, Yoon-bok heads to speak with Chief Censor Kim in prison. Prince Yi-yeol had tasked him with investigating Gil-dong's true identity. He is shocked to find Chief Censor Kim murdered, with the name "Gil-dong" written in blood on the floor.
Later, Eun-jo receives distressing news from a servant at her new home about the death of Chief Censor Kim. She is further shocked to hear rumors that Gildong was responsible for his demise. She donns the Gil-dong mask and sets out to uncover the truth. Unfortunately, she is spotted by the police bureau and narrowly escapes their grasp, slipping and falling from a rooftop. She manages to throw her attire into a well before collapsing.
At the end of the episode, something mysterious occurs. Earlier, a monk had singled out Eun-jo and Prince Yi-yeol, giving them matching bracelets after they offered alms. As Eun-jo lies unconscious by the well while Prince Yi-yeol is at the palace, the bracelets begin to glow. They both fall unconscious and wake up in each other's bodies.
This scene, rich with emotional turmoil and intrigue, leaves us eagerly anticipating what lies ahead for our protagonists as they navigate this unexpected turn of events.
