Just checking that you've set the default to A4 in System Preferences>Print & Scan?
If so, and you've still got the problem, I wonder if a 'reset of the printing system' will help:
Yes I have tried a full reset and also deleting all printers and downloading latest drivers, sadly all of which had no effect.
My latest link was to change the language setting to Britich english and delete the English as it was suggested this had an afffect however it has not changed anything.
All defaults appear as A4 in both printer and system prefs but when you open a word/excell/pages doc and try and print the paper size comes up as US letter.
Any help is much appreciated as this has been going on for months and is driving me mad.
I'm afraid I'm equally flummoxed - it would drive me nuts as well!
I don't use any Microsoft products but I do use Pages and it behaves for me as I would hope - A4 every time.
If I can find any suggested fixes I'll be back. In the meantime, hopefully some others may spot this thread and have some ideas.