I doubt that Apple is going through this silly numbering 4, 5, 6, etc.
Now what I really am expecting is that next year it would be the same design plus the official 4G coverage, that being said, I think they will call it "iPhone 4(insert letter here)". But, the fact of the network upgrading, obligates Apple on making a bigger upgrade on the phone in mostly every aspect so that users have more than just simple
spec bumps. Chances are minimum though since according to Apple, the iPhone 4 is a big leap in general, making that silly for them if next year they are going to introduce a whole new iPhone.
And yes, I think I will keep my iPhone 4 for two years, unless the next one has twice of the battery life : D