The logic doesn't work. They used '3G' because they were releasing a 3G phone. If there's any connection at all the next phone will be the 4G, but for a number of reasons I doubt they'll head in that direction. It will probably be iPhone 5.
No, you've missed it.
What he's saying is, in the past, when Apple has used only a number as the model designator following the iPhone name, that number has represented the generation of the phone. Granted, this has only happened with the iPhone 4 so far. But it only stands to reason that it will continue in the future. And, of course, the other naming pattern they've established is that an "S" clearly indicates that the new generation phone is an enhancement to the previous generation phone (retaining essentially the same form factor).
The next generation iPhone will be the 6th generation iPhone. So, if they decide to just give it a numeric designation, iPhone 6 is the only thing that makes sense. Calling it iPhone 5 would be just as arbitrary as calling it iPhone 1, iPhone 3.14, or iPhone 666, or for that matter, iPhone Q, iPhone =, or iPhone schwa, since it is neither indicative of the model's generation nor of anything else.
Now, if they decide to name the phone based on some criteria other than the generation, then plenty of other possibilities exist, sure. Perhaps iPhone 4G or iPhone LTE to indicate it supports 4G (LTE) communications (if it does), or maybe iPhone 4Pro, or iPhone 4Plus, or iPhone super-duper, or whatever.
I think there's a very good chance we'll see the tapered iPhone in about a year with the larger screen and home button similar to the leaked prototype. My guess is they won't call it the iPhone 6, but something more meaningful. iPhone 4G might be the ticket. We'll see.