Hi Archie:
MachIV is special, it is under development as a hardware pulseing engine. It is to be ignored by all but testers until such time as its fully released.
Mach2, and 3, ad simply the early, and the recent. Mach3 is an ongoing project of adding features, and adding capabilities. Read all the MAch3 docs for specifics, but most of us are now usng Mach3, as its much more powerfull.
Regards,
Art