I have picked up this little breakout board.  http://www.dmm-tech.com/Break-outBoard.html

I want to be able to control the spindle RPM with one of the axis but at some point I may want to add a rotary 4th axis to the machine.  Should I try and find a 5 axis breakout board or can I simply set up two mill profiles, one with the X axis controlled normally and another with the x axis data directed to the 4th axis (since I would probably not used the X axis and 4th axis at the same time)?  I guess another possibilty would be to control the spindle VFD through a seperate control of some kind.

Any thoughts?

You probably can't control spindle speed with this type of bob.

You'll need a 0-10 volts analog output and the model you have do not have this option as I can see.

Almost all PWM controlled spindle controlled need a 0-10 volts analog voltage.

Hope this help, Jeff