Hi,
this is a common fault especially when first starting out with Mach4. If Mach4 does not shut down completely and gracefully
then it is likely that part of it stays running, commonly the GUI chunk, and that prevents another instance from starting.
When I first started with Mach4 I managed to crash it regularly and had to use Task Manager to shut down the offending parts
before I could restart. I haven't had to do that for three to four years now.
I suggest focus on what you are doing to cause Mach4 to crash....and fix that....then you need no workaround.
Craig