In your Poser directory in the Textures you should find the textures for the P4 man and woman in the P4Textures folder. A good way to start is by taking this in PaintShop and pasting the photo of the face you want on the map. Make sure you line up the face correctly, you may need to render and try it first. Of course other changes might be necessary to make the colour hue of the face match that of the body and so on. I don't use PaintShop though so I don't know how it handles layers, etc, but there is a forum for it check on the right.