I have a MKX-4 card. the 4th axis works only as slave. At any other options an axis of 4 stop.

I just received this drive and have found that the problem is with the fact  that these controllers will only control 3 drives at once.. X/Y/Z, X/A/Z but, not 4 physical axis can be used.. I do not believe there is an adjustment or fix that can be done..  *sorry*..