I'm with AT&T and yes it is a 4g phone, when compared to other current 4g phones. Originally 4g was for 100mbps and we are not even close to that so they changed the current high speed internet to "4G" (but no one realy cares about that now).
With AT&T, if you have 4G LTE in your area (very limited in US) you will get 4g speeds. The signals swithces from 4G LTE to 4G HSPA+ and down to 3G HSPA when needed with 4g LTE being the fastest. Other carriers don't have this option. The Thrill uses all three networks under AT&T.
See more detail, AT&T coverage area, and how 4g works on ATT phones below.