What is Unknown Error 4000 ?

Well there are, at least, four reasons why you would see this error, and they are:

1. The iPhone already has the latest update.
2. The iPhone is locked with a passcode. It needs to be unlocked during the update process. To prevent it from locking automatically, tap on Settings > Display & Brightness > temporarily set Auto-Lock to Never.
3. There is a newer update for iTunes for Windows. (Looks like you got this one covered already.)
4. The phone's battery has less than a 50% charge.

If none of those work, try:

• Restarting both the phone and iTunes.
• Uninstalling iTunes for Windows, and then, reinstall it again from the Apple site.
• Try using another USB cable.
• Windows Registry errors.

Here’s the REAL issue:

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. So 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.

Your iPhone does not need a Wi-Fi connection when connected by USB, but your Windows 10 PC must be connected to the Internet either by Wi-Fi or Ethernet for this update to work.