One definition of 4G combines the use the 3G with Wi-Fi.
The definition of 4G, at least the marketeers definition, changes depending on which marketeer you talk to. One definition that's gaining some traction here at the 3GSM World Congress in Barcelona is the combination of WiMax with 3G (more about that in an upcoming blog).
A second definition, and one that I'm more in favor of, is the combination of 3G with Wi-Fi. The way this technology works is that if you're in a hot spot (which includes your home or office), the phone works over Wi-Fi, using VoIP. If you're any place else, it connects over the 3G network.
While roaming the halls at the show, I just witnessed a demo of this technology. The demo was put together by PCTel, who developed the front-end software to allow the handoff between the two technologies. While the user does notice the handoff, it occurs fairly gracefully. There's a short delay, generally less than five seconds, whereby you don't hear the party at the other end. Not perfect, but definitely acceptable.