Error code 4000

Error 4000 is an undocumented error code (at least publicly undocumented). It means that the connection between the phone and iTunes was interrupted in the middle of a sync or update activity. In the past this might have been caused by a flakey USB port, an antivirus or a firewall. However, there is a new cause of this error that is the result of a bug in iTunes for Windows that requires the phone not be locked during the update. I think this has been finally fixed in the latest iTunes update, so try getting that from https://itunes.com/. If not first check your antivirus and firewall (temporarily disable them), but if that doesn’t fix it there are three workarounds for the bug:

• Update over the air, by going to Settings/General/Software Update
• Downgrade iTunes to version 12.8 if you can find a copy - try https://apple.com/itunes
• Temporarily disable the screen lock passcode on the phone before updating using iTunes on Windows

Note that this error should only be a problem for updates; if you are restoring iOS rather than updating it should not occur.

You shouldn’t have antivirus on your phone. If you have Windows you should definitely have an antivirus. But turning off automatic locking won’t work, because the phone will still lock when it restarts. If you don’t want to update over the air (or can’t) you will have to disable your passcode entirely. Or see if there is a newer version of iTunes, or go back to iTunes 12.8.

Well, that’s a first. Error 4000 is specifically a code that means that the physical connection between an iPhone and a computer failed. Are you doing the update with the phone plugged in to your computer, for power, perhaps?