The phone is called iPhone 4, not iPhone 4G. If it was, it would be on a 4G network. AT&T only has a 3G network now. Sprint is the only carrier with 4G at the moment.
AT&T is in the process to begin rolling out their LTE (4G) network which should begin the first half of next year in some select major markets only initially, but the iPhone 4 is not an LTE (4G) network compatible phone.