Where do people refer a "2x4" as a "4x2"? Sincere question.

I have heard it, but only in the way of inferring that it was being used in an abnormal or "flat" orientation, such as for a floor truss assembly

I believe most carpenters in the UK would still say '2 by 4' and '2 by 1'. Odd, as those values are pretty much meaningless, as they refer to the original size of the timber before it was planed down. In this sense, those dimensions have now become terms.