Whilst you can't use V4 skin textures for V3 because of the different mapping, you can transfer any material shaders (for example those that come with a V4 character you have purchased) on to any other figure, using the target figure's original texture maps (colour, specular, bump) plugged into the new shader.

You could do this manually. But bagginsbill's VSS freebie (Versatile Shader System), once it has been set up for use with V3, would enable you to do this with a single button click for each V3 character you have.

Or am I still missing the point?  :O)

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)