Although the trilogy of games starring Geralt of Rivia may be the last we see of the white-haired monster hunter, in a new interview (in Polish) with CD Projekt RED's Adam Kicinski, we learn that The Witcher represents one of two franchises that will shape the future of the developer. And that the next game in the series would not be called The Witcher 4.
As translated, the full statement is as follows:
"The first three Witchers were by definition a trilogy, so we simply could not name the next game The Witcher 4," Kiciński replied. "This does not mean, of course, that we will leave the world of The Witcher, created by us for over a dozen years. The Witcher is one of two franchises on which to build in the future activities of the company. Today, unfortunately, I can not reveal anything more."
Which adds a little flame to the idea that the next entry in the series will say goodbye to Geralt as protagonist and present a new story as told from the perspective of Ciri. Either way CD Projekt RED is definitely invested in the franchise as it continues to develop and support The Witcher-universe card-game Gwent in addition to creating a table-top RPG set in the universe.