The problem is the "External Binary Morphs" introduced with Poser 6 and the way V4 is constructed. The problem has been fixed in Poser 7 SR1 - but of course not in the old P6. So in order to use V4 with Poser 6 you need to disable the use of external binary morphs. And that will only help with new scenes of course. Poser 5 will open those files as it doesn't know anything about the PMD files there the morphs are stored when external binary morphs are enabled. Also deleting the .pmd file will also make the file readable with Poser 6. Both Poser 5 and 6 won't display the V4 morphs then, though.