Let's start with you giving some basic details to stop people giving random answers :-

What version of Windows are you using ?
What directory is P8 installed in ?
What choice did you make for the content directory when installing P8 ?
What directory is V4 installed in ?

Bonus questions
Have you successfully used V4 with a previous version of Poser (if you have one or more) ?
Have you added the Runtime with V4 in using P8 ?