In my opinion there's no chance The Witcher 4 will flop. It will most certainly sell very well.
It might not have the same cultural impact The Witcher 3 had on the world for so many years, but it sure will be well sold initially.
Depending on the quality of the gameplay and quests, it might reach 3's fame.
I personally can't see a scenario where The Witcher 4 hurts GOG.
Its a The Witcher game after The Witcher 3 with Ciri. A The Witcher game, with Ciri.
With Ciri. ...with Ciri, get it?
Modding possibilities, big world exploration.
Ciri... see? Ciri, modding, world, quests, Ciri. :P
The Witcher 3 launch apparently was very bad, and they fixed it.
We could argue that the people who made 3 is not there anymore, but CD Project Red has a history of terrible launches and fixing their games through time.
The only scenario where 4 hurts GOG is the one that The Witcher 4 is so bad, with so much bad writing, destroying Ciri character to a point of no return and bland gameplay that people would call it out, which is, to me, impossible of happening.
Why impossible? Because The Witcher 4 is a RPG.
There will be many choices, quests, and so on, with bad and good outcomes. Devs already explained that they intend to give players in game reasons and player choices for Ciri development and how she will, or will not, become a Witcher, maybe, continue to be, or not, a Witcher.
They also said Geralt will appear at some point, so there will probably be choices also based on their interaction. Coming from 3, where you could choose how to deal with Ciri's will to be her own and to prove herself, this will probably also impact how Ciri on 4 will deal with CD Project Red interpretation of how Geralt would treat Ciri, now, the player, on being a witcher or living a life so hard as the witchers.
Again, it might not top 3, but there are plenty talented people on CD Project Red still.
I don't think it will flop at all.