You can't have both programs opened at the same time, but you can have them installed to the same folder with no problems. I have P4 w/PP in the same folder and P5 accesses Poser 4's library with no problems.I use Poser 4 a lot yet, but render in Poser 5 with Firefly. I also have python scripts for lighting that will only run in P4 which I don't have the $ to upgrade right now. I have passed a file from one program to another with absolutely no problem. Eventually I will probably uninstall P4 and just retain it's runtime library. I still store all my 3rd party items in P4, because while P5 can access P4's library, P4 can access P5.