The iPhone "xyz" could not be updated. An unknown error occurred (4000) Using iTunes to update my iPhone, I got:
Das iPhone &quot;xyz&quot; konnte nicht aktualisiert werden. Ein unbekannter Fehler ist aufgetreten (4000)
English:
The iPhone “xyz” could not be updated. An unknown error occurred (4000).

I could not solve it with the Apple recommended answer at I got Error 4000 when updating iPhone asking you to:

update iTunes
update the pc
plug in the cable directly (I have not done anything else anyway)
Restart both pc and iPhone
Switch off antivirus and firewall in all possible options

How to fix this?
Just update your iPhone using your iPhone update function, not the one offered in iTunes.
I have overseen another possible solution in the link of the question:

&quot;Finally just put iPhone into Recovery Mode and did an update.
SUCCESS.  It did the 12.4.1 update and all seems fine.&quot;

Probably that would have helped as well. But the easiest solution is just to update your iPhone as you would do anyway if you did not have iTunes.