I just read a fantastic interview in Vanity Fair of Lucas, Spielberg, and the man Harrison himself. They talk all about the new movie, a bit about the old ones, and a little about themselves. I loved it.
Spoiler Warning (i.e. don't read on Sir Steven--I know how you hate to know anything about movies before hand)!!!!
So, the fourth movie is set in the late 50s and has more of a science fiction twist to it. Add to that the emergence of a new villain (for Indy at least) of the Russians, and I gotta tell you that I'm stoked! I had a ton to say, but then I realized that the article says it all for me, so I'll just leave it at that: go read the article.