I am not too concerned. The first Iron Man opened up the same month as Indy 4 and still made money. I think Cap and Potter are different demographics anyway.
Yeah, but Iron Man opened up several weeks before Indy 4 and already had well over $200 million in the bank when Indy 4 opened. There's a very real possibility that Captain America won't win its opening weekend. Not that it's necessarily a death knell, Sherlock Holmes did very well competing directly against Avatar, but it sure would make it harder to get attention in the marketplace.
I don't really think Potter and Cap are different demographics, simply because of how consistently large Potter's box office is. You don't do that much unless you appeal to all demographics, including the demographics that most frequently go to the movies.
But, that's life in the big leagues. There's competition out there and you have to rise to the challenge.