I tried to update my IPad on my new computer on I Tunes and get the error 4000.

Try updating over WiFi. If you don’t have access to WiFi, try turning off your passcode on the iPad and then try updating again. When you turn off the passcode, you will lose all of the cards in Apple wallet and they will have to be entered again.

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 Contact 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