Season 4 of Haikyuu begins approximately at chapter 290. But keep in mind that there might be some variations depending on different translations or editions.
The ending of season 4 of Haikyuu in the manga is at a point where a major plot development occurs. It's a bit complex to pinpoint exactly, but it's related to a key match or event in the story.
It ends at a specific chapter that focuses on a crucial match and character development. But I can't recall the exact chapter number off the top of my head.
The start of Haikyuu Season 4 in the manga is approximately at chapter 290. This might not be exact as sometimes there are differences in how the content is presented and divided between the two mediums.
The plot of the 16th episode of the second season of the year of celebration revolved around Fan Xian's return with the diplomatic mission. The Second Prince threatened Fan Xian with the safety of Fei Jie, Fan Sizhe, and the Teng family's orphan in an attempt to force him to lower his head to him. Fan Xian faced the Baoyue Brothel maze and the spring examinations crisis. These were all traps set by the Second Prince. In addition, Fan Xian also took over the palace treasury. He discovered that the palace treasury was heavily in debt, so he decided to rely on his own power to resolve the palace treasury crisis. Fan Xian and Lin Wan 'er held a grand wedding, but then the Qing Emperor suffered three assassination attempts. Fan Xian's rescue resulted in his martial arts being completely crippled.