My iphone 12 won't update, unknown error 4000

See this Apple article: If you see an error when you update or restore your iPhone, iPad, or iPod

If you see an error when you update or restore your iPhone, iPad, or iPod - Apple Support (SG)

Connect your device to your computer using an Apple USB cable & try also another USB cable.

Turn your iPhone on and unlock it -> enter the passcode.

Go to Settings -> Display and Brightness  -> Auto-Lock set to Never.

Guess you use a Windows PC, if so check the AV & Firewall you use on the PC. 

Disable all AV & Firewalls temporarily while you restore & update your apple device.

Turn Off Hotspot.

Turn Off or remove VPN if necessary.