In the Modify Selection part of the Grrouping Tool is the Add Material and Add Group. You use them both. First the Add Material to all all the materials of the head (minus the ones you want visible) to a new group you've made in the Head part. This is the one I called INVIS in my example.

Once you've added all the parts to that group, you click the Assign Material and make a new material, I called mine INVISIBLE (I like naming my own special material zones in all caps so I can easily locate them. It is of course optional)

Next you select the neck and eye GROUPS (and more if you want it) with the Add Group button and once they're selected as well, you Assign Material to them as well, again the INVISIBLE material.

When all is done, you add a 100% transparent, no colour, no highlights material to the INVISIBLE parts. And upon rendering, you will see only the mouth. In OpenGL there's a faint shadow of the rest and I believe Poser 4 uses a sort of dithering patttern on transparencies IIRC, so you may see a ghosting on the parts you've hidden, but they won't render.

FREEBIES! | My Gallery | My Store | My FB | Tumblr |
You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.