what is error code 4000

Two alternatives:

Keeping in mind that this workaround can affect keychain and/or Apple Pay (if in use, you might need to re-input data), try these steps to solve error 4000 on your iTunes:

• Disconnect your device from the computer
• Temporarily disable the passcode on your iPhone
• Restart the iPhone
• Restart iTunes
• Reconnect the iPhone to your computer and restart the update procedure.

When finished, re-enable your passcode on your iPhone.

or

Update your iPhone wirelessly (via wifi).

Regards

Giulio