I've heard that P4 creates a RSR for each OBJ file (as a quick loading version of the OBJ file). These RSR files apparently sometimes become corrupted (especially if Poser crashes) and need to be deleted for the figure to load. The RSR files in question will be located in the geometries folder beside the original OBJ file.

While I've seen this info posted several times, I've never actually had this problem (I use P5) so I can't guarantee it'll work :(

Hope this helps.