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.
"real 4g" haha, that's a good one. It's all just marketing speak.