iphone couldn’t be updated unknown error 4000

Update bug, check this https://forums.macrumors.com/threads/trying-to-update-to-ipad-os-14-itunes-giving-error-4000.2255178/

Hello aakash196,

Thanks for that info and for choosing the Apple Support Communities. If we understand correctly, you are unable to update your iPhone with your computer, as you get an error message. To help with this, make sure your iPhone is unlocked when you start the update process, and follow the relevant steps in these Apple resources:

iOS update and restore errors

If you see an error when you update or restore your iPhone, iPad, or iPod

Cheers!