P5 will not overwrite P4, and you will need to keep the P4 install if you use the PP plugins. In P5, you can link to P4 libraries, but it may have trouble finding geometries and textures. When this happens it gets confused and sluggish, and if you don't point to where there file is, it farts and dies. I run both with identical runtimes, it is less of a hassle, easy to synchronise, and I always have a backup. The only downfall is the amount of space it takes, but with 60GB devoted to Poser, it isn't much of an issue. I rarely use P4 anymore except to use the plugins, in which case the files need to be in the P4 runtime.