CD Projekt RED isn't developing The Witcher 4 (it's too busy with Cyberpunk 2077 for one thing), but if they do decide to make it, then maybe they should have a listen to Geralt voice actor Doug Cockle's ideas about a possible sequel.
Chatting with Gamereactor, Cockle was asked if Geralt's guest stint in SoulCalibur VI would be the last time he would voice the gruff hero, which lead to the following response:
'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. In The Witcher 3 she hints at visiting other worlds, and I think a game around her visiting those different worlds would be a cool Witcher 4.'
CD Projekt RED has been pretty vocal over the fact Geralt's story would only last three games, and was neatly wrapped up in the absolute monster hit that was The Witcher 3: Wild Hunt.
Still, the potential for a Witcher 4 is definitely there, and if nothing else, financially it would probably make a lot of sense; after all, the franchise has sold over 33 million copies worldwide.