This is not strictly a Mach3 vs Mach4 thing. It is the way PMDX appears to have designed their devices vs. the way the Mach3 parallel port (appears to have) behaved when Mach is "disabled". It looks like maybe the PMDX-411 drives the output signals low when Mach4 is disabled, even if it is configured as active low. Perhaps they figured this was s safest option. Or perhaps it was an oversight handling the enable signals.