Ah... iPhoto 5's database is incompatible with iPhoto 4, and it sounds like when you imported the photos with iPhoto 4 it overwrote (or "updated") it's own database to the new version. If this is what happened, I don't know of a good way to "roll back" the database to the older version...
If there are no photos in the old iPhoto 4 library that you wish to keep, you can simply delete the iPhoto index file in ~/Pictures/iPhoto Library. Then, try dragging-and-dropping the photos into the iPhoto 4 library, which should cause it to rebuild the database in it's own format.
You might want to make a backup of the iPhoto Library folder before you try this just in case you'd like to be able to salvage any photos already in iPhoto 4 when you tried the import.
Just guessing, though -- that's what it sounds like to me.