The reason I asked why you'd set the step pulse width to 5 in my first post is that you don't need to change it from the default 0 as far as the G201 is concerned.
The 201 steps on the falling edge and its min hold time is 0.5us. The Step Pulse setting in motor tuning is NOT the step pulse width, it's the time you want to ADD to the stock step pulse width that Mach puts out. Art says somewhere in one of his vids that this is between 1 and 2 us - so already 2 to 4 times greater than that needed for the 201.
However - You don't say anything about your BOB (if you use one). Some BOBs have Slooooooow optos and if so these can require longer step pulses from Mach. (one of the reasons I don't use BOBs).
You don't say what the current requirement is of your system BEFORE adding the 4th axis so whether the 4th is taking it over the limit of your PS is not possible to say.
All that said: you say in reply #9 that your power supply is 23Vdc. Chopper drives will not work below a "certain" voltage and the 201's min is 24Vdc so that could well be an issue.
EDIT: forgot to add that your 3.8mH motors "ideal" voltage for torque at speed is 62Vdc so in that respect also your power supply is not a good match.
Ian