I have often said the exact same thing. He is comparible to the names mentioned above - at least during the past 50 or so years. Even more, the movies that he's certainly helped define during that time will also be the ones mentioned throughout history while most others have been long forgotten. Indy 4 will be on that list.
While on the subject of the score - it is interesting to speculate on some of the potential new themes that will be included in the new movie.
All the Indy movies have a certain formula - as do their scores.
such as...
The Raiders March, obviously.
Then there's the theme for the main villain and/or artifact (the Ark, The Grail, Mola Ram, The Nazi's in Last Crusade, etc.)
Then there's the more emotional or romantic themes (Marion, Willy, Short Round, Henry Sr.)
Then you will always have at least one central suspense or action theme in each film. (The truck chase, the slave children's crusade, the boat chase, the motorcycle chase, the tank scene, etc)
I can't see Spielberg or Lucas encouraging JW to change this formula for the fourth and probably final installment.
My guess is that you'll have a theme for Shia Lebeouf's character and Cate Blanchett (for whichever role she plays - villain or heroine)
The main artifact or quest will get a theme. It's a sure thing.
The villians will have some sort of march or choral piece - maybe something similar to the theme that was cut from Attack of the Clones and put back into Revenge of the Sith. Or even something like the Sunday Night Football theme that Williams created for NBC. (I can't help but think of Indy when I hear it.) A Russian or KGB theme would be a nice addition.
If Marion is in there - you know they'll bring back her theme. Even if only for a minute.
The Raiders March will be used as it was in Temple of Doom and Last Crusade. You may hear alot of it in whatever opening action sequence - it's been almost 20 years, after all.
You'll probably have a more humorous action theme (like the basket chase) for the scene in New Haven that they've been talking about. Maybe with a little late 50's flair.
Then, like the tank scene, you'll probably have a much more serious and suspenseful action theme. Which will lead to the finale.
And again, I pray that there is no reference to Yoda or E.T. in this new score. If it's true that life exists on other planets - Keep it out of the Indy movies!!!