That shouldn't be a problem if you're within the 30 days buyer's remorse time frame. Just take it back and tell them you were not satisfied. Your contract should reset for upgrade eligibility, then go back and purchase a new iPhone.
You could purchase a refurbed iPhone 4 for discounted cost over a new one (I believe $150 for new contract or upgrade). If you paid the full price for brand new, you should expect a brand new product.
wherever you bought it, if your within your 30 days then all you say is you want an exchange. You return your current phone, and then you re-buy a new phone all at the same time (it might take just a few minutes for your account to re-set with AT&T), I did this 2 weeks and I was in and out in 20 minutes (The Apple Store was really busy)