> 4.1 never ran on classic, AFAIK.

All OS4 releases since November 2007 including current 4.1FE Update 1 have been available for PowerUP-equipped classic Amigas.

> The best use for a BPPC/CPPC setup is still to use AmigaOS3.9, WarpOS
> (with PowerUP emulation). Unfortunately you don't get later developments
> like MUI4. The retro boom unfortunately didn't result in a boom in 68k ports

MUI4/MUI5 for OS3/m68k: https://muidev.de/downloads :-)