I've come across a number of free items that state "P5 only" and work perfectly well in P4. The only things that are genuinely P5 are those that have dynamic cloth/hair or are specific to P5 characters, or consist of shaders only. In the case of a prop that is supplied with P5 textures, it's trivially easy to add your own non-P5 textures, especially if you are rendering outside of Poser anyway. Therefore "P5-only" doesn't apply. I would look at it like this - the originator hasn't tested it on P4 and is just covering themselves by limiting it to the system for which it has been tested.