I should clarify what I meant.
"4G" has two meanings: the fourth "generation" of Apple's hardware, and its also the name of the "next generation" of cellular data and voice communication networking.
These two things are not actually related.
So yeah, we might get a "4th gen" iPhone as early as this summer, but I would expect it to be basically the same as the 3GS, just a little more/better.
I was referring in my previous post to an iPhone that was able to use "4G" cell networks, which are only just now starting to roll out. That is what I think will take a couple of years to appear (figure summer 2011 at the earliest).