"The first three Witchers were by definition a trilogy, so we simply could not name the next game 'The Witcher 4'. This does not mean, of course, that we will leave the world of The Witcher. The Witcher is one of two franchises on which to build the future activities of the company. Today, unfortunately, I can not reveal anything more," said Kiciński in an interview with Polish outlet Bankier (as translated by Google Translate).
USGamer
Not that it should come as much of a shock to anyone, but CD Projekt RED has confirmed that it will make a new game set in the Witcher universe, and that it will not be The Witcher 4. Not only that, they explicitly confirm that The Witcher will be one of their two main franchises going forward (I assume Cyberpunk will be the other), meaning I imagine a Bethesda style Elder Scrolls/Fallout routine for a while.