Ahh I see we are confusing terminology. Yes the phone is showing 4g because it was designed for the t mobile network where 4g is really enhanced 3g. Att has "true" 4g where it is on LTE. Essentially when you are using an ATT sim you are getting 3g from them which the phone recognizes as 4G. This is because T mobile 3G and 4g are really the same thing and they use 4g as a marketing tool to compete with people that have real 4g.
I am surprised that the phone gets att 3g because the specs show it as only t mobile 3g bands. Can anyone link to where it says it might be a quad band 3g phone?
Att also calls their HSPA+ 4g. My wife's iphone 4S showed 4G on ATT when connected to HSPA+.