This is not just about the iPhone either. There are a lot of other phones claim to have 4G that don't really have it, although, the iPhone is quite possibly the farthest away from real 4G standards as it doesn't even support AT&T's HSPA+24Mbit/s network, it only supports the 14.4Mbit/s version of HSPA. Here is a little explanation about what 4G is supposed to be.
This is going to get a little technical, so if you don't know what Mbit/s or Gbit/s keep in mind that a typical home connection ranges from 1Mbit/s to 40Mbit/s.
To be designated as a 4G network you need download speeds of at least 100Mbit/s while moving and up to 1Gbit/s in stationary applications, such as on a house or a person walking with a device. Upload speeds are in the requirements too, but I'm leaving them out to simplify things.
Under this definition, LTE is the closest thing to 4G out there since it has a theoretical max download speed of 100Mbit/s meeting the minimum moving requirement. However, it is theoretical because it is unlikely that you ever reach that speed due to outside interference; this holds true for every wireless technology. LTE's second iteration (much like HSPA's move to HSPA+) will offer theoretical speeds of 1Gbit/s, true 4G, but that is a little ways in the future.
HSPA+'s max theoretical possible download speed is 672Mbit/s, although it has never been implemented by a US carrier. Currently T-Mobile has the fastest HSPA+ variant that operates a 42MBit/s while AT&T's network tops out with the 21Mbit/s variant even though, the iPhone's radio they now claim is 4G maxes out at 14.4 Mbit/s.
To be simple in terms of G's they should be sort of like this.
- AT&T iPhone: HSPA+ - 14.4 Mbit/s- 3G
- AT&T HSPA+ : 24Mbit/s - 3.4G
- T-Mobile HSPA+ : 42Mbit/s - 3.6G
- AT&T/Verizon LTE :100Mbit/s - 3.85G