If you are trying to update your iOS device (iPhone or iPad) using iTunes, you may receive an error message after the iOS update downloads and attempts to install to your device.
The iPhone “” could not be updated. An unknown error occurred (4000).
The error most likely indicates your iPhone is already updated with the version of iOS iTunes downloaded. Verify this on your iPhone and attempt the updated using your iPhone by going to Settings -> General -> Software Update.
It is also possible your iOS device locked with a passcode during the beginning of the installation in iTunes and this caused the update to fail.
If you still want to update your iOS using iTunes, ensure you are using the latest version of iTunes from Apple. Reboot your iOS device and attempt the update through iTunes again by plugging your iOS device into your computer.