I've only seen the fourth one (gasp!) but would be interested in this. Most people say the fourth one is the weakest and I thought that was good. But not a "must get" for me unless it's at a wicked discount.
I thought the 4th one was ok, but it focused too much on showing off Indy's son. Which i hear he may be the star if they do future Indy movies. The 4th movie had some charm of the originals, but not that much. It was nice to see the original gal from the first movie in it again.
I can't believe you've only see the last one.