Apple iPhoto v4.0.1 Update is Out

and available for download via the Software Update preference pane if iPhoto 4.0 is installed. There is also a standalone updater available from this Apple web page. According to the Read Me file:

Photo 4.0.1 includes many organizational and stability enhancements. Performance has also been increased throughout the application, giving you faster importing, smoother image viewing and easier Rendezvous photo sharing. This version of iPhoto also features improved thumbnail images in your photo library. To take advantage of this, your thumbnails must be upgraded, which may take a while, depending on the number of photos in your library.

Updating the thumbnails of our meagre four hundred photos took 4-5 minutes.