I never uninstall any of my Posers because I like to test things for compatibility, troubleshoot problems for fellow users who haven't upgraded yet, and create version-specific MATs. So I have no way of knowing whether uninstalling one version might break another.

Idle speculation:

If P5 installs system files which P4 also requires, those files might have been removed when you uninstalled. But that wouldn't explain P6's continued operation, unless the latest version of Poser no longer requires them.