what is error code 4000?

This is a Windows iTunes error. For right now, until Apple provides a fix to iTunes, a workaround is to temporarily disable your passcode on the iPhone and it should work. However, I will tell you that I have starting using Over the Air updates. You won't see the error there.

I've shared an article about resolving iTunes error codes below. The article provides some great steps to troubleshoot general iTunes error codes. However, Code 4000 is not listed. I recommend following each of the steps in this article in the order specified. Also, be sure the lightning cable you are using is a genuine Apple cable. It may be worthwhile to try another Apple cable first and then try another USB port with a known working Apple cable, both of which are covered in the article shared below. If you are using a non-Apple cable then using an Apple cable may resolve the issue for you.

iOS update and restore errors - Apple Support

How are you doing the update? Over Wi-Fi or through iTunes?

You're welcome.