4x5, if you mean 4 pixels by 5 pixels, then there would be 20 pixels in total.
Though that is a very, very small image, so maybe you mean aspect ratio; 4:5.
Aspect ratios don't have a specific pixel count, because just like in mathematics, an aspect ratio is just the ratio of the screen's width to the screen's height.
For example, a 40x50 monitor (don't take it literally) has an aspect ratio of 4:5, but then so does a 400x500 monitor, so pixel count is not consistent across aspect ratios.
