Keep in mind, though, that the iPhone 4 is no longer receiving operating-system updates from Apple. It still will work, but if there's a need for an OS security update or if newer versions of Pandora or a favorite app require newer versions of the OS (Apple will be at iOS 9 not too long from now), you're outta luck with a 4. A 4S or 5/5c/5s still will work, and it seems iOS 9 will run on anything that could run iOS 8, so that's as old as I would go.
I'm sure there's a counterpoint in the Android world, especially with the layers that most phone manufacturers add to Android. And I know Windows Phone has been through a few iterations that left existing phone owners behind. Just something to consider in evaluating the purchase of a used phone.