The keywords "the Witcher season 4" are trending primarily due to the recent release of the new season and a major change in its lead actor.

Here's the background and reasons:

• "The Witcher" is a popular fantasy series on Netflix, based on a series of books and video games. It tells the story of Geralt of Rivia, a monster hunter known as a Witcher, and his adventures in a magical world.
• A major cast change for Season 4: The biggest reason for the trend is that the main actor, Henry Cavill, who played Geralt of Rivia for the first three seasons, has been replaced by Liam Hemsworth for season 4. This change was announced in October 2022, and has been a significant topic of discussion among fans ever since.
• Reasons for Henry Cavill's departure: While Henry Cavill has not explicitly stated his reasons, reports suggest it was a "symbiotic decision" with the showrunners, partly because he had other projects he wanted to pursue. There are also rumors that creative differences regarding the show's faithfulness to the original books contributed to his exit, as Cavill had previously expressed a desire for the show to be more "book accurate".
• Anticipation and mixed reactions to the new season: With the release of Season 4, viewers are eager to see how Liam Hemsworth performs in the iconic role. Early reviews and fan reactions have been mixed, with some critics finding Hemsworth's portrayal to be "fine" but requiring time to get used to, while many fans on social media have expressed disappointment, calling his Geralt "soulless" and "lifeless" compared to Cavill's. The way the show introduces Liam Hemsworth's Geralt into the story has also been a point of discussion.
• Netflix's release strategy change: Unlike previous seasons, which were sometimes split into multiple parts, The Witcher season 4 was released all at once, which is also drawing attention as a shift in Netflix's usual strategy for big shows.

In essence, "The Witcher season 4" is trending because a highly anticipated new season of a popular show has arrived with a new lead actor, generating a lot of curiosity, discussion, and strong reactions from its fanbase.