This long into a product cycle, I don't know why anyone would buy an iPhone 4.
Maybe because iOs 4.x and time has crippled their aged iphone 3G to the point that it was painful to use on a daily basis. Maybe because they see a lot of value in recording day-to-day memories on a video camera they always have with them, and similarly take better pictures.
And maybe, because they're not in the least bit convinced that the iPhone 5 revision is going to add much of value (4G isn't going to be useful for a while yet until the network support is there, and the 4 isn't lacking in much else, frankly). The iPhone 6 may be better poised to take advantage of the faster networks when they're more widely deployed, and starting a new contract now means you're actually closer to upgrading naturally when the 6 is out instead of having to wait for the 7.
Seriously, the 4 solves pretty much all of the complaints I had with the previous generations. It's as responsive as I had always hoped they would be, the camera is good, battery life, etc, etc. If they never improved it from this point on, I'd honestly be ok with that, whereas I wouldn't have said that about the original, 3G, or 3GS. Now that AT&T has a sensible 1000 texting plan, that even solves the other gripe I had with them.
As another poster said, it's a phone that also has a lot of other useful functionality, and has matured in this evolution very, very well. But yeah, IT'S A PHONE. Buy it whenever you want it, enjoy it, and don't look back.