What is your iPhoto version? On a new mac I assume iPhoto'11?
How did you migrate your iPhoto library to your new iMac? I do not see how you could possibly get four copies of everything, if you copied the library to your new mac. This looks like you now are seeing previews, edited versions, thumbnails along with your original master files. Did you import the iPhoto library instead of simply copying to the new mac and opening it by double clicking it?
If that is the case, I'd recommend to copy your library from the old mac to the new mac, and this time simply open it in iPhoto, do not import it, and forget the corrupted library. That might be the fastest and safest solution.
But if you cannot start over, and have to fix your current library, you have to find a way to delete the photos in small batches. iPhoto can only delete a small number of photos at a time. To be able to do that you have to remove the photos from the trash and put them back into an event. Only, if you use the "Put Back" command, iPhoto will move them back to their original events and you will have to track them down again. The only event that you can drag to is the most recently used event in the "Recent" section of the source list. So I'd suggest to try to turn a nearly empty event into your your most recent event - import a new photo or something, and then to drag all your photos from the Trash to this recent event. Only drag a few images at a time - no more than 1000 or so.
When your trash is empty, quit iPhoto to save this state. Then open again and start deleting photos from the recent project - no more than 1000 photos at a time. Take care to empty the trash immediatly, each time you add photos to the trash.
Regards
Léonie
