CD Projekt RED has been pretty vocal about The Witcher 4 not being in development. But, like the fans of the game, it sounds like a member of the cast of The Witcher 3 wants a sequel, and they want it to focus on Ciri.
Speaking to Gamereactor, Doug Cockle (who voices Geralt) was asked about whether his upcoming cameo as a guest character in Soul Calibur 6 would be the final time that he portrays the character. "If you're leading into The Witcher 4, I can only say I have no idea, although I think it will be. This is my own opinion, and CD Projekt Red knows what they are doing, and I have no idea what the plan is, but... If I was CD Projekt Red, I would make The Witcher 4, but I would be focusing on Ciri."
Cockle goes on to point out that in The Witcher 3, Ciri "hints at visiting other worlds", with Cyberpunk 2077 being one of them. "I think a game around her visiting those different worlds would be a cool Witcher 4", Cockle concludes with.
So, The Witcher 4 definitely isn't in development, but it's leading star certainly has a wish for the series to continue, and for it to focus on his adopted daughter. This actually isn't an uncommon wish in the fanbase for The Witcher series, as Ciri proved to be one of the best characters of The Witcher 3, if not the entire series.