for photography, aperture is measured with the f/ratio. a f/5 is a f/5, doesn't matter if it's 1mm or 1km in diameter.
if u do the maths, u will realise that the 4" f/5 will have a focal length of 500mm, and the 6" f/5 will have a focal lenth of 750.
convert all these to photographic terms, we are comparing a 500mm f/5 to a 750mm f/5. exposure times will be same, but image size is different.
~MooEy~