lgr
I have a 4th axis on my machine, and it seems to work quite well, its not super fast, but its a lot faster than 200deg/min.
Of memory it runs a revolution every 10secs.
I dont really have much advise to offer, but if you like I can send you my XML file and you can look through it to see if and what differences are between yours and mine, especially in the configs and motor tuning.
My 4th axis is my B axis as it runs parallel to the Y, and I'm using CNC4PC bo.boards, I cant see why or what could be at fault.