Update Error code 4000

This support page makes reference to 4000-series errors:

If you see error 9, 4005, 4013 or 4014 when you restore your iOS device – Apple Support

Whilst Error #4000 is not explicitly listed in this support page, Apple Support are known to recommend that you follow these steps when troubleshooting this error - prior to seeking further assistance from Apple Support.

Apple Support can be contacted directly using the Get Support link at top-right of this page - or by using the excellent Apple Support App. If the App is not already installed, it can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

You have 2 solutions:

1. If you connect your iPhone to iTunes via the USB cable, unlock the phone
2. If you are trying to update your iPhone, try using Wi-Fi instead without using the cable (Settings > General > Software Update)

My mistake, I should have been more specific: you need to remove the lock.

Please see the following User Tips for more info: https://discussions.apple.com/docs/DOC-250003773

Did you unlock the phone before connecting?

Excellent !!! We all have to thank Lawrence Finch for this great User Tip. He is a precious resource on this forum.