Welcome to APUG!

You put your first post in the perfect place.

Like many things photographic, the Rodagon lenses have changed over time. It may be that you are asking about lenses from different eras. I have a set of older Rodagons (including 50mm and 105mm lenses, but not 80mm) that are f/5.6 and are quite good, but my much more modern 80mm f/4 Rodagon is equally good, and more convenient to use.

That being said, one of the practical differences between an f/4 lens and an f/5.6 lens is that the f/4 lens will be brighter at maximum aperture. This can make focussing and composing on the easel much easier, especially with colour negatives.