AFAIK, there is no firmware yet to control a 4 axis CNC in our 3D printer univers.
I'm afraid you'd have to program the kinematics for a rotating axis yourself. My best bet is the Duet3D.com board with RepRapFirmware.
Will you drive the XY-axis via belt or leadscrew? IMHO, the first is not suited for heavy milling, the latter is to slow for 3D printing