4G (HSPA+) has been available in many areas for over a year. It depends on your location. Seeing as many areas are not even on 3G, its a safe bet that those same areas will not have any form of 4G anytime soon.
If you are on 4G/HSPA+ you should see an "H" or "4G" icon in the notification bar (not sure which is the case for the Inspire stock OS).
You can also test your upload/download speeds with Speedtest, a free app on the Android Market. Do a few tests at different times of the day to get a good average, as network speed flucutates greatly due to traffic. I think AT&T was claiming "up to 5 Mbps" speed on the HSPA+ network, when fully deployed. But I've seen anywhere from less than 1 Mbps up to 4 Mbps at the same exact geographic location(my work). Network speed at my house is really dodgey in comparison (despite being a highly developed suburb) often with speeds less than 0.5 Mbps, when I can get data reception at all. Luckily, I am always on WiFi at home.