The fourth chapter primarily delves into the past, yet we begin in the present. Issei and Camille face off for a photo shoot, their rivalry having become a major talking point in Japan, with the press capitalizing on their popularity. Meanwhile, Hirokazu has gone missing, and Honoka is sent his belongings in a large envelope. Inside, she discovers her husband's wallet, phone, and even his wedding ring. There's also a piece of paper, presumably a suicide note for the cold wife. As the reality of this note starts to set in, viewers are taken back to 1990. A youthful Honoka and Hirokazu travel to France to visit Chassangre's vineyard. Alexandre makes a rock-star entrance, delivering a wine-tasting tour for wealthy Japanese tourists.
Honoka's reserved, cutting persona is evident from the very beginning, and she challenges Alexandre instantly. She requests to taste some rarer, more exquisite wine. Alexandre picks out the perfect alternative, and Honoka likes it so much that she orders 50 crates of the stuff to be delivered back to Tokyo. At a party that night, it's clear that Honoka has eyes for Alexandre, but nothing comes of their initial flirtations. A bashful Hirokazu attempts to move on Honoka, but he is quickly turned down. Honoka appears to be envious of Alexandre's beautiful wife, Marianne. Then they all have a grape fight together.
A year later, Alexandre decides to move to Japan to become an oenology teacher in Tokyo. This drastic move is spurred on by Alexandre's aversion to the current French wine industry, which he thinks is outdated and elitist. Feeling like an outcast in his own country, Alexandre and Marianne take a risky leap of faith.
Their first few weeks in Tokyo are difficult. They have no money, and Alexandre's course starts off poorly. He has a minimum of students interested, and they cannot drink wine in class. Alexandre decides to teach them after hours at a wine bar instead. The students love the experience and quickly learn so much. Marianne gets a job working at the French Embassy, and she introduces Alexandre to the publisher Jacques Fusier at a party. Again, Alexandre's strong, controversial views offend this wine expert as well.
Back in the present, Marianne is sent Alexandre's ashes in the post. The urn comes with a message from Alexandre himself. He apologizes for not supporting her or making her happy. He wants his ashes scattered at the Chassangre vineyard.
While in Tokyo, Issei confronts Honoka about her past with Alexandre, showing his mother the photograph of them together. Honoka downplays their interactions, clearly hiding some secrets of her own. Honoka asks Issei if he has seen his father. The son has not. She keeps her worries about Hirokazu's fate to herself, though.
In further flashbacks, Alexandre and Honoka's simmering romance starts to heat up. Alexandre starts to walk Honoka home after their wine bar seminars, but they don't take things any further. Hirokazu sees the couple together and gets rather drunk, jealous of their relationship.
The next day, Hirokazu is discovered dozing off outside the university gates. News of Alexandre's extracurricular activities with the students spreads like wildfire, and he is swiftly dismissed from his position. Marianne urges Alexandre to write his own wine guide, now that he's unemployed. She arranges a meeting with Jacques, where Alexandre presents his revolutionary vision for the wine industry. Jacques reluctantly agrees to a book deal, offering him a cash advance to sustain himself. Alexandre dives into work on his first wine guide, which will soon become a global sensation.
Honoka arrives at Alexandre's home to deliver Hirokazu's apology letter. Sparks fly between them, and they start a passionate love affair, meeting regularly in hotel rooms. Alexandre juggles both romances and still finds time to complete his wine guide. Marianne falls pregnant, and soon after, Honoka becomes pregnant too. It's not explicitly stated that Alexandre is the father, but it's a strong possibility. When Camille is born, Alexandre also spots Honoka and Hirokazu leaving the maternity ward with Issei. They were born within days of each other.
Back in the present, Honoka flips through old photographs, reminiscing about her time shared with Alexandre. Marianne hides Alexandre's ashes, refusing to fulfill his final wish of having them scattered in Chassangre's vineyard. Issei and Camille share a lift together, presumably heading to their next challenge. Are they half-siblings?
(Note: The story you've provided contains some vague elements that are not fully explained, such as whether Alexandre is the father of Honoka and Marianne's children. I've tried to maintain the essence of the story while making it more expressive and vivid.)
