Apple no longer sells the iPhone 4s on the Apple Store but that doesn't mean that you can't find it in stores or online. It's still widely available and marketed as a budget option. It's also a device that we don't think you should buy now or at any point in the future unless you really, truly don't have any other options.
The iPhone 4s was recently upgraded to iOS 8 but that could be the end of the road for a device that was released in 2011. Apple tends to keep devices updated for three years before kicking them to the curb (the iPad 2 is an exception) so we may not see another upgrade arrive for the iPhone 4s.
That's not the only reason it makes this list. It's also on here because the iPhone 5c and iPhone 5s are both extremely cheap and both of these devices are better than the iPhone 4s. We recommend both of those over the iPhone 4s because of the hardware upgrades and due to the longevity.