Just to expand a little on what ockham said: Keep them separate. P5 can use multiple runtimes. So install P5 separately, then add your P4 runtime to P5's. You can do this by selecting a Libarary, clicking up the heirarchy til you reach the top, (which, after initial install will show you "Poser 5" (with a red ball next to it) and "Downloads"), then click the "+" button at the bottom. Navigate to your P4 Runtime and add it. P5 will add the new Runtime to all of your library categories (Figures, Props, etc..) Cheers, Diolma