How do I enable content auto-saving within CKEditor 4?

There are many ways to automatically save editor changes. You can do the following:

On editor blur use the editor#blur event.
Once every N seconds use editor.checkDirty() and resetDirty() to verify whether changes have been made.
When a user types, use the editor#change event, however, it's not recommended to make AJAX calls on every #change event as it may happen too often. The event must be debounced e.g., by using the eventsBuffer method.

You can also search our add-ons repository for a plugin that allows for saving and restoring content automatically.