You can make Posette with V4 head. Only disadvantage would be gape/scar on neck where Posette and Vicky polygons touch each other - I think you cannot weld it and make smooth joint.

1 - Set Posette in "0" pose. Set her head invisible.
2 - Pose V4 placing her head where Posette head was. Use scale if necessery.
3 - Using group tool make prop with V4 head. If you scaled it better export/import V4 head obj (DO NOT check "make polygon normal..." in import window)
4 - Hide V4 body (make her invisible). You should see Posette with New Prop in place of original head. Now select Posette head and in "object" meny choose "replace body part with prop.
5 - You have Posette with Vicky head but eyeballs are in wrong place. Repeat step 3 and 4 with eyeballs.
6 - Go to setup room. Switch camera on right or left and make jaw bones (they should have no geometry atm). Check V4 bones in setup room and try to place Posette jaws in similar position.
7 - Repeat step 3 and 4 for jaws.
8 - Try to move Posette new eyes. Probably they do not work well. Open "joint editor" and fix center of eyeballs bones in correct places.
http://www.renderosity.com/mod/forumpro/media/folder_9/file_413186.jpg
Just in case "joint editor" window and parameters which should be fixed.

This figure should accept V4 morphs in morphmanager. If morphs cause weird results try export obj "as a morph target" from V4 and import to new head.