Ed Dig up a copy of Post Exposure By Ctein. He has a chapter devoted to enlatging lenses and testing. I agree that an enlarging lens is just like a camera lens. They are just designed to work at different magnification ratios. It is NOT my opinion that enlarging lenses that work well in the 4-5.6 range are poor at all other stops. In fact some of them work fairly nicely wide open. I would also imagine that many photographers that work with glassless carrriers may, when there is a goodly amount of detail at the edges and corners may prefer an aperture of at least f8. I neither fault no criticize anyone for whatever methodology they want to use.

How much difference f4, for instance, there is with f8, for the second instance, is going to also depend on other factors. How much details is in the negative, how well is the enlarger aligned, what is the light source, how big is the resulting print are some of the factors involved.

It is worth remembering that with an 8x magnification that the effective f stop is f 36 if the lens is set at f4.

As far as the S-Biogon is concerned it does not have an aperture. Secondarily, both lenses were desinged to have their best performance for reproduction rations from 15-70x. This is well outside of normal parameters.
I have not used either lens. Zeiss states that these two lenses, used at these magnification ratios, will significantly out perform anything on the worlds market.

I have high regards for both Leitz and Zeiss....among others.