Arnaud wrote:But you didn't ask the question for 4 and 40...
I googled 40. The explanation seems logical. There are times when I have no problems accepting "that's the way it just is".
Googling 90 gave me the "ten to hundred" explanation, which doesn't make any sense; nine but (or to) hundred is 91, not 90... but apparently it has nothing to do with но сто, and then I can accept it It's funny how one's mind works.
(And I know how to count in French and Danish... some interesting ways of forming the numbers there )