like daniellyall, said,
example:
How are you driving your spindle motor?
Step/Dir? (through what motion device?), over Modbus? (what functions?, what bit resolution), through some other method?
What functions are you wanting to control?
i.e. On/Off, CW/CCW, Enable, fault reset, speed profiles, read spindle speed, read fault(s), etc. to include reading a speed encoder if there is one.
There is A lot of possibilities so BE SPECIFIC in your questions!
and having said, the above, there may not be an answer, since perhaps no one is using a set up like yours,
and has nothing to test against........ also, Mach4 is still in Beta.......
Scott