One thing I believe semi-hurt Indy 4's box office was the advertisement. Frankly, I think burying the trailers until Valentine's Day was not a smart idea. I think they should've done what they did with the Phantom Menace and truly promote the hell out of it. It would've had a bigger opening weekend with everyone officially pumped that Indiana Jones is BACK. All there was was the concept that sold the movie, and obviously, the big grosses prove that Indiana Jones is a truly beloved name that can still sell close to the top. But if they wanted to achieve TDK numbers(which is what I predicted in 2007) they should've elaborated the importance of May 22nd, 2008 to the entire world and assure those stupid "But Harrison is too old" people.
Still not sure why they didn't go with that plan. Maybe Lucas didn't want fan's expectations to haven been up even higher?
Now, about Indy 5, it should still make $300 million no question, and while they sort of missed their opportunity with the campaign on Indy 4(just a little though), if they wanted bigger #s, a bigger campaign in addition with a weak summer like 2009(where they don't have a movie like TDK) should do it.