Wow, it's pretty big jump from 4.5 to 4.0 than I thought.
Going to be honest..
I also was thinking it was no more than the 500 rpm/Volt between 4.5 and 4.0
There seems to be another 1000 rpm/Volt increase to 3.5 being 10100 rpm/Volt
But then 2.5 is 2 hp or something like that, funny old world hey
Alex