I don't think the cross runtime theory holds up. When I reinstalled Poser 4 I Copied the (Non P5 specific) runtime files over to P4. Uninstalled P5, and did a fresh reinstall, and Patched it to SR3. I have had Zero problems using the P4 Runtime nearly exclusively in P5, and though I didn't do much with her in P5, I did Open V3 in P5 (installed in P4 Runtime.) with no apperant problems.