When trying to update my phone via Itunes I get an error code 4000, any ideas what is causing this

Error 4000 is not on the error list and is a bug in some versions of itunes for Windows.

First update iTunes.

If it still errors then

Either

(1) Use Wifi instead, Settings, General, Software updates

or

(2) Use itunes but temporarily disable passcode on the phone. That will clean out any cards you have in Wallet 

or

(3) borrow a Mac