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The reason why telescope makers use fluorite and PFL-53 glass is because of their dispersion value off the abbe line. To get better colour correction you need to use at least one glass that deviates from the abbe dispersion line, and fluorite makes a good match to the readily available and cheap BK7.
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I forgot to mention dispersion, thanks.
Unfortunately fluorite it's not cheap, otherwise we'd all have PFL-53 glass in our refractors!!
Anyway....I was comparing like with like, not the ED with the 6" SN. Naturally the 6" would outperform the 4"....on all accounts.
BTW...that glass made by Schott, is called "Schott Glass"....it's just a manufacturing name, as you'd already know. However, the best glass ever made is a "shot" glass....with a wee drop of the good stuff to go with it
Just like my take on the "Galileoscope"....called the "Gallianoscope", and you get to see plenty of "stars" with it as well