Don't know much about how Mach4 deals with plasma cutters. That may be a better question for the Mach4 support forums. I don't see a Mach4 output signal specifically for "torch on" (though there is an "torch on" INPUT signal). You could probably use the "Spindle On" output signal to turn your torch on and off. Depending on what type of signal your plasma current needs to turn the torch on (voltage levels, drive current) you could use the PMDX-416's relay contacts or one of the TTL level (5 volt) outputs.
For example, to map the "Spindle On" signals to the PMDX-416's relay, go to the Mach4 "Configure" menu then select "Mach...". When the dialog appears, click on the "Output Signals" tab. Scroll WAY down until you see the "Spindle On" signal in the left column. Click the red "X" next to that to enable it (green check mark). Click in the "Device" box and select the PMDX SmartBOB. Click in the "Output Name" box and select "Relay". Leave the "active Low" as a red X. The "Spindle CW" or "Spindle CCW" buttons and M3 or M4 commands should now turn on the relay. M5 should turn it off.
