The definition of 4G is fairly loose, to say the least. Every carrier pushes their own version. Sprint was first, with WiMax. Then T-Mobile started pushing HSPA+. Verizon launched LTE, the fastest. And then AT&T started tossing around "4G" as well.
However you want to define 4G (or not), what is clear, as Sascha Segan points out, is that AT&T is currently peddling the worst 4G phones of anybody. In PCMag's testing, AT&T's two "4G" phones, the Atrix and Inspire, are much slower than some of their 3G phones, like the iPhone 4 and Dell Venue. AT&T admits it's deliberately turned off HSUPA on the Atrix-effectively capping the Atrix's upload speeds-even while it continues to push the device as a 4G phone. (Not that AT&T is alone in over-promising and under-delivering on 4G.) And while Moto's Xoom on Verizon may only be 4G-capable in the future, at least it's relatively up front about that fact.
For all the rumblings about the FCC trying to implement and enforce net neutrality, what I'd love to see in the short term—and what seems eminently doable—is some concrete regulations about promised (or implied) data speeds and what consumers actually see on the other end of the pipe. In other words, if AT&T—or anyone—calls a device 4G, it should be delivering specific minimum average data speeds, as long as network conditions permit it. (I wouldn't expect 4G to melt my pockets into my flesh sitting in the middle of the Super Bowl, for instance.)