The shape of the 4 makes it much more stable and easy to hold, plus the flat, square edges make it perfect for using as a spirit level, for resting on a flat surface to take time lapse photos, or just to stabalise it for longer exposures. It’s also much stronger having a stainless steel frame, instead of plastic. I broke the plastic back on my 3Gs by dropping it onto the headphone jack. That’ll never happen with a 4. The only way the 4/5 could be really improved is for the frame to be cast from Liquid Metal™, and a carbonfiber back put on. There’s no way Apple will ever go back to the 3/3Gs form factor, they need all the space available to cram in a big battery, memory, CPU/Graphics, 8Mp camera, phone radio, Bluetooth, wifi…