Currently, each class has 4 at will possibilities to choose from. Of those, they choose 2.
If every class every example get to eventually choose 4, they every copy of that class would have the same at-wills. And as levels get higher, you gain more encounter and daily powers, which you should be using more than your at-wills.
That's a solid argument, but frankly I think WOTC should have added more than 4 at wills!! I mean, that's not a lot of customization right there. I'm sure more at wills will be added in later books, but 4 is just a really low number when you choose half of them at 1st level.