Turn UAC off. It is not your friend.

Using the V4 installer, install to a runtime that is NOT under Windows' "Program Files"

Install V4 to the runtime. If the installer cries about not finding "Poser.exe", make a text file in the runtime base directory and change the name to "Poser.exe"

MAKE SURE V4 INITIALIZES. Even if it says it did, click on the little batch file on your desktop.

If you have not already done so, add the runtime in Poser.

You ought to be in business.

BTW... turn UAC off. UAC is not your friend.