AT&T is going call their HSPA+ (normally considered fast 3G), 4G, like T-Mobile. They're also going to start deploying their LTE network (Verizon's type of 4G) starting this summer. For an explanation between the two, see here.
Unlike T-Mobile, who only have HSPA+, or Verizon, who only are going to call LTE "4G", AT&T is going to call two different technologies "4G". I could see that getting very confusing, explaining to people that they can't get this type of 4G here, but this phone supports that type of 4G.