The solution is easy.  Your V4 installer is looking for 'Poser.exe' which doesn't exist in Poser Pro.  V4 was released before the Poser Pro version.  Open your notepod program and create a txt file.  Type something, then save.  Rename that file Poser.exe and place it into the Poser Pro folder where 'PoserPro.exe' resides.  Reinstall your V4.  It should see 'Poser.exe' and install correctly.