Thanks to all of you for your reaction and comments.
It is not about the user preferences. It is about the printer default. Look at the "Printing Defaults" on the Advanced tab of the printer properties.
IMHO that is written in HKLM.
I think the answer is somewhere in this key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Print\Printers\Microsoft Office Document Image Writer
The binary entry "Default DevMode"
Starting at position 66 the word 
"L e t t e r  "
is written.
When changing the "Paper Size" in the "Printing Defaults" to "A4", this binary value changes starting at position 66 to 
"A 4  "

I tried to change this in the registry, but apparently this was too simple...