Why does my iphone 4 show 3G instead of 4g when I pay for 4g on tmobile family plan?

I get 500 mb of high speed data then after the 500 mb of data it goes to 3G but my iPhone 4 just shows 3G. The whole T-mobile plan is unlimited texts calls and data but the first 500 mb of data is 4g, is the data speed the same but it just shows the 3G symbol or is my phone not supported for 4g

The iPhone 4 is not a 4G phone.