There is of course the discussion
https://www.mobileread.com/forums/sho...d.php?t=174234 that you were party to.
In essence, as was said there, conversion may be the way forward.
I may do something stupid like attempt building a few 4 and 5 bits for a 3 one day but don't hold your breath.
Hope that helps somewhat.
Think "Calibre"...
https://www.mobileread.com/forums/arc.../t-175820.html
cheers...