My dust extractor is located outside my workshop and switched manually from inside the workshop. For a long time now I have considered switching it on/off from within a GCode file in a similar manner to the spindle.
By using a simple HID, USB, relay I can override the manual switch and control the extractor from my PC’s, USB port, from Mach3, without any direct connection to my CNC machine electronics.
Although I have only tried this using Mach3 I am sure the same thing can be done using Mach4.
The USB relay I have used cost less than $7 (inc. postage from China) and woks like a charm.
For greater things, these HID configured relay boards are available with up to 8 relays and of course, more than one board can be used (including with a USB hub).

Tweakie.

Only when I came to make some recent modifications to an engraving machine I found another one of these HID relays had been fitted (a couple of years ago) by my bro. Paul to switch the machines dedicated vacuum pump on/off using modified M3/M5 command scripts. I noticed he had also made a beneficial modification by replacing (disconnecting) the on-board mechanical relay with a SolidStateRelay which makes for low noise, smoother (zero crossing) switching of a.c. loads.

Tweakie.