To reply to this older post: It's as easy as going to File > Page Setup and in "Format for" select "Any Printer" (or a specific printer), and in "Paper size" you obviously select A4. This should be a permanent setting, but you can still change print settings ad hoc.

wuergler - I don't what operating system you have, but on Windows in Firefox there are no preferences for Paper Size in Page Setup. That setting needs to be changed thru the Properties button after doing File > Print.