What is iTunes error 4000 caused by? How to fix it?
This error appears when you tried to update your iOS device via iTunes, but the update failed to complete.
The most common causes of iPod touch, iPad, or iPhone could not be updated. An unknown error occurred (4000) are:
Before proceeding further, make sure to check your device if it has already been updated to the latest version, because sometimes iTunes might fail to notify you about that (due to the reasons mentioned above). Also, ensure to stay by your PC all the time, while the updating process is taking place, as you might be asked to provide your passcode or fingerprint.