Just the other day I upgraded from a 3GS to a 4S.
My reason for skipping the iPhone 4 was that I felt it's lifespan was just about over. Just like the 3GS, the iPhone 4 didn't get all the features of iOS 6 like 3D maps and panorama photos, and it lacks things like Siri and 1080p video recording. The iPhone 4S on the other hand has all the features of iOS 6 that the iPhone 5 has so they are pretty much the same phone with just a few hardware and design changes. Also, I took into account how many future versions of iOS will be supported on the device; the iPhone 4 might not get iOS 7 whereas the iPhone 4S will probably survive till at least iOS 8.
My reason for skipping the iPhone 4 was that I felt it's lifespan was just about over. Just like the 3GS, the iPhone 4 didn't get all the features of iOS 6 like 3D maps and panorama photos, and it lacks things like Siri and 1080p video recording. The iPhone 4S on the other hand has all the features of iOS 6 that the iPhone 5 has so they are pretty much the same phone with just a few hardware and design changes. Also, I took into account how many future versions of iOS will be supported on the device; the iPhone 4 might not get iOS 7 whereas the iPhone 4S will probably survive till at least iOS 8.