There's no Witcher 4 planned, but we will "probably" return to the franchise's world
So yes, The Witcher series will live on, but not as another Geralt story.
Developers at CD Projekt Red have been hopeful that the studio would return to The Witcher at some point in the future, but right now, the studio is hard at work on another game. That doesn't mean that there's no chance for the series to make a comeback though.
According to Kiciński, CD Projekt Red isn't planning on making a Witcher 4, as they always saw the series as a trilogy, but they do still own the IP. Kiciński said that in the future we will "probably" be returning to the world of The Witcher, just not necessarily as Geralt.
No further detail on the subject was given.