Mach 4 will not use the DDE method like Mach 3 did. So whatever their software did for Mach 3 will not work for Mach 4. Mach 4 has an inter-process communications (IPC) library that could be used to accomplish the same thing. However, that would require that Vectric compiles against this Mach 4 IPC library. All I can say is I don't believe they have contacted us about doing anything like that.
That being said, the next build will have a command line interface for loading files. When Mach4GUI is running, simply calling "Mach4GUI -o <gcode file path>" will load a file. If Mach4GUI is not running, it will launch Mach4GUI and load the specified file at startup.
Steve