Superhero movies have been coming out since the 1990s in decent number.
http://en.wikipedia.org/wiki/Category:1990s_superhero_films
Blade: Trinity, Elektra, Fantastic Four 2, Ghost Rider, The Incredible Hulk, Punisher: Warzone, and X-Men Origins: Wolverine were all not huge hits. They made close to double their reported budgets on Wikipedia or less than that.
With the movie rights to the Fantastic Four, Spider-Man, X-Men, and the Hulk not with Marvel when Disney purchased them, that's many of Marvel's most popular characters or groups gone at the time of purchase. Additionally because those characters/groups and others like Daredevil and the Punisher had already had movies made featuring them that takes away from some desire to make movies featuring them if Marvel got the rights back even while under Disney's ownership. So some of the appeal of making movies with Marvel's most popular characters at the time of the purchase is not as great as it might otherwise be if superhero movies were just as popular at the time of purchase somehow, but those characters hadn't appeared in movies yet when Disney purchased Marvel.
Given all that, the Marvel superheroes they might reasonably expect to have in movies in the near future weren't traditionally as popular as the ones I mentioned although some of the remaining did have a long history in the USA in comics and tv shows and there is some appeal to having characters like that. There were still various other superheroes and other action hero comics Disney could probably have got the movie, theme park, and/or merchandise rights from various comic books companies for something like the comic book section of Islands of Adventure if they wanted and at a lesser cost. There were also more recently made characters like Delilah Dirk which Disney could have got the rights for (which they have done for Delilah Dirk). Additionally, there are public domain super heroes and action heroes from comics. Although, using public domain characters could allow other companies to make merchandise and other movies with the character.
I'm no expert on this stuff, but to me 4 billion seems like a lot of money given the circumstances. Disney paid much, much less than that for CrossGen, although they were much less popular and bankrupt. There is the appeal of stopping Universal from possibly purchasing Marvel or otherwise minimizing the benefit they might receive from future Marvel movies and new theme park attractions they could put in there, but look at the financial track record of movies featuring Marvel characters before the purchase. It doesn't seem to me like there was any very obvious reason to believe that many future Marvel movies at the time would be very financially successful and be very popular. Even Thor 1 and Captain America 1 that were released after the Disney purchase weren't big hits theatrically. Disney might have believed in themselves and Marvel Studios in obtaining success with this stuff more so than they thought many other studios would have done, but it's not like Disney or other studios haven't had a decline in their general financial success in making movies at one time or another or had financial failures in movies that they even put a lot of money in.