There are games that have so many distinct endings that sequels have to pick some as "canon" and drop others. But it looks like CD Projekt RED has found a way to treat all of The Witcher 3: Wild Hunt's endings equally.
In an interview with IGN Franchise & Lore Designer Cian Maher confirms that all endings in The Witcher 3 are treated as canon in The Witcher 4, even the one where Ciri apparently dies:
"The one complication is probably the idea that there is an ending in which Ciri can die in The Witcher 3. There are hints in that ending that highlight the fact that she probably does not die. The Witcher 4 isn't going to break any canon or even offend any canon"
Of course, we don't know exactly how the game picks up from the third game, but we do know that Ciri is the main character and that Geralt is still there.