From what I can gather by a little research I do not believe that it is a bug in DS 4 Pro. at all, to put it simply files that are required for the shaders to work are not there.
From what I have found.
A shader in Daz Studio should be composed of:
a shader definition script, linking the different files bellow
a RSL surface or light shader (.sdl), which is a compiled version of a .sl surface/light shader code.
a script defining the shader properties, which means also the user interface in the surface tab
a render time script run every time the shader is called with attributes passed.
As far as I can see there are no shader definitions for the toon shaders in the DS4 folders and also in the DS4 shaders folder the dztoonskin.sdl is non existent.
It would be nice to have them but I fear we are not going to have such a privilege, it is a business after all.