GoodNotes 4 will still be supported for an extended period of time and will receive bug fixes and minor improvements so that you can continue using the old app.
The current Mac version has been removed from sale on the App Store for now to avoid any confusion.
We're working on a new version for GoodNotes for Mac, that will be compatible with GoodNotes 5. It will be available soon. If you're interested in beta testing the new Mac app pre-release, please sign up for our waiting list. We'll invite you when we start beta testing it.