ios update to 15.
I keep getting this error 4000 right after the backup portion of my iphone update.
Windows, Windows 10
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)
Guess you use a Windows PC, if so check als the AV & Firewall on the PC.
Disable your AV & Firewalls temporarily while you restore & update your apple device.
If they are On, turn Off Hotspot & VPN.