According to Movie Moron (there's a name to inspire confidence) the plan, way back in the 80's was for five movies. But after the third one, there was disagreement over the fourth script and everyone moved on to other projects. So, theoretically, we're owed another movie.
Aside from that moron, there's been other chatter - according to an October 3rd, 2008, LA Times story momentum is building for a fifth movie:
"It's automatic, really, we did well with the last one and with that having done well and been a positive experience, it's not surprising that some people want to do it again," Ford said.
In discussing a possible Indy 5, it looks like Mutt Williams may go the way of Jar-Jar Binks, as an idea that didn't quite work out:
This past May, two weeks before “Indiana Jones and the Kingdom of the Crystal Skull” opened worldwide, George Lucas told reporters at the Cannes Film Festival that he already had an idea for a possible “Indiana Jones V,” and that it centered, not on Dr. Henry Jones, but on his son, Mutt Williams (Shia LaBeouf).
Three knife-flipping, tree-swinging, accident having months later and that idea? It’s kind of, well, “Nuked the Fridge,” Lucas told MTV News.
Asked whether he still considered Mutt Williams a strong enough character to drive an “Indiana Jones” film, Lucas was adamant that Indy just isn’t Indy without Indy. Or, to put it another way: No.
“Indiana Jones is Indiana Jones. Harrison Ford IS Indiana Jones,” Lucas said, dismissively adding about a character he helped create that, “If it was Mutt Williams it would be ‘Mutt Williams and the Search for Elvis’ or something.”
In the meantime, I've discovered that there's something of a cottage industry online in fake theatrical trailers for fake Indiana Jones' movies. I thought this one struck the right blend of suggestion and humor:
This one was based on the Legos' Game. While I've never been very big on computer games, this one looks like fun:
Finally, this one is so well made, that I think it really would make a great Indy movie! George, Steven - anyone listening out there?