Here's the update from CD Projeckt Red's Witcher website page:
MARCH 21ST, 2022
We’re happy to announce that the next installment in The Witcher series of video games is currently in development, kicking off a new saga for the franchise.
This is an exciting moment as we’re moving from REDengine to Unreal Engine 5, beginning a multi-year strategic partnership with Epic Games. It covers not only licensing, but technical development of Unreal Engine 5, as well as potential future versions of Unreal Engine, where relevant. We'll closely collaborate with Epic Games’ developers with the primary goal being to help tailor the engine for open-world experiences.
At this point, no further details regarding the game — such as a development time frame or release date — are available.
REDengine, the technology which powers Cyberpunk 2077, is still being used for the development of the upcoming Cyberpunk 2077 expansion.