If you have a modelling application - and I think you should be able to do this in Blender - select the door (with luck there will be some kind of select by part or group command that will let you pick the whole door part from a list in one go), cut to clipboard and paste back in place then save. The problem is that the mesh is welded in the OBJ and in Poser (and DS3 and earlier) split apart into groups on import, but DS4 doesn't do that any more so any welds in the OBJ remain in the imported model with the result you see.