âYou change to a 600/4. Your exposure is the same because you are still f/4.â
This is the only point I was trying to make. 500mm or 600mm the total amount of light across the frame is the same as long as you are at f/4.
What you seem to be saying is that more of the light that is gathered is on your subject rather than wasted on the background.
You can get that effect by using a longer focal length or getting closer to the subject. Either way you increase the amount of light covering your subject and avoid unnecessary enlargement therefore reducing visible noise.