The ideal way would be to install Poser 4 and Poser 5 in separate directories. Then add Poser 4's runtime to Poser 5. Now install everything new into Poser 4's runtime folder. You can delete unused or duplicate content like the P4 and P2 figures from Poser 5's runtime, but do not delete the whole runtime folder. There are files in e.g. runtime/ui, runtime/prefs, runtime/plugins, runtime/python etc that are vital for Poser and not interchangeable between Poser 4 and Poser 5. File compression: You don't have to compress manually. ProPack and Poser 5 come with a Python script that can compress whole folders and subfolders. And I think Ockham wrote a script once that deletes redundant files, e.g. .rsr files for which pngs exist or cr2s when there are czzs in the same place.