a mat pose is not a standard poser file, its a hack, so you cant save a mat pose directly. You can save a material collection in the materials room. 

If you plan on using V4 with your skin often just save to the figures library with a new name.  Load the figure when you need it and the skin will be on it already.

 

once you have an mc6 file , edit it with wordpad(or editor of choice) and change the line that reads mtlcollection to figure $current and save it in your pose library as a pz2 file.  Copy the thumbnail for the mc6 to the pose library too.   

 

(I think thats it)