So many people have questions regarding how PWM works for spindle control in MACH4.
I'll give you a few details.
The PWM signal has a duty cycle, which is a product of the commanded S value vs the overall spindle speed for the SPINDLE RANGE you're in.
Duty cycle is simply the percentage of active time of a signal over a given time period.
So if your max spindle speed is 10k RPM (declared in the spindle tab, CONFIGURE > CONTROL > RIGHT ARROW OVER TO SPINDLE TAB)
Assuming a max speed of 10k, when you command a speed of 5k you'll have a 50% duty cycle on your PWM signal.
If you question is regarding the lower frequency ranges permitted by PWM in general, I'd have to check with someone but I'm pretty sure the limitation is strictly hardware based.