I believe that v4 should have resolved issues that some users were experiencing with v3 (http://brettgrossphotography.com/2008/03/04/aperture-applescript-make-keywords-real-v03) crashing Aperture.
Make Keywords Real
Current version: 04
Built on: Aperture 2.1 / Mac OS X 10.5.6
This AppleScript operates on the current selection.
This is an update to the previous update and should work properly now.
This AppleScript will make Aperture’s virtual keywords [...]