Figure morphs are added body part by body part. You are getting that error message because Poser is comparing the number of vertices in the currently selected body part with the number of vertices in your full V4 morphed OBJ.

There are a number of utilities and applications out there that will help you with this. Details of the one I made are at http://www.philc.net/PTB_page3.php Scroll down to "Load Morphs from OBJ"