I also recall when I worked on a European-designed typesetting machine, the language had me specify the vertical dimension first, followed by the horizontal dimension. So when I would created a 9 1/2 inch x 11 inch continuous form design (that ran on a web press with perfs at every 11 inches), I had to key the size as 11 x 9 1/2.
So my next question would be, when you specify 5x4...
Are you doing it because you are specifying the long dimension first?
Or are you specifying the height first?
No, regardless of portrait or landscape we say 5 X 4, or as a previous post mentioned, more often 5,4.