I put in one of the iPhoto CDs that I made with iPhoto2 and it showed up in iPhoto4 without a problem. On a whim, I selected all the albums on the CD and just dragged them to the bottom of the album list; I saw the green plus sign and dropped them. The import took some time, but to my amazement, *everything* was preserved! Albums, rolls, dates, originals, everything. Now I finally have all my photos in one place. I hope this works for others also.
You should, of course, make backups before trying this, but you did that before you installed iPhoto4, just like Apple told you to, right!?

