CKEditor 5 is a new concept of rich text editor. Compared to CKEditor 4, every aspect of it has been redesigned - from installation, to integration, to features, to its data model and its API. Therefore, moving applications from CKEditor 4 to CKEditor 5 means migration rather than an upgrade. Users can decide to move from CKEditor 4 to CKEditor 5 at any point of time depending on their use case.
Please read the CKEditor 5 and CKEditor 4 comparison section to see which editor fits your use case best.