Posers 5-6 have a feature called external runtimes. You're no longer limited to the single set of library folders within Poser; you can have runtimes anywhere, even on other hard drives or removable media. And after you tell Poser's library where to find these runtimes, you can switch from one runtime to another at will.

You don't need to move any of your old P4 stuff, unless you plan to completely uninstall P4. You can simply point P6's library to the existing P4 runtime, and use it from there.

jelisa has a tutorial on the subject.