I am not sure how this '3GS was a bigger step' conclusion can be reached.
The 3GS brought:
- ~200MHz faster processor
- 128 MB more RAM
- digital compass
- voice control
- improved rear camera (50% more pixels)
- ??
The 4 added the following:
- 200MHz faster processor
- 256 MB more RAM
- Retina display - double resolution, and also IPS
- front camera
- video calling
- improved rear camera (50% more pixels plus a much better sensor)
- LED flash
- gyroscope
- noise canceling microphone
- better voice quality and reception (except in areas of poor reception + death grip).
I do agree about the durability issues, but feature-wise the iPhone 4 was a massive leap that added every missing, oft-requested feature plus some.
The parts that you are missing are the changes regarding the architecture of the hardware. The 3G was using the old ARM architecture (same CPU that was in the original iPhone), and a very slow GPU. The 3GS moved up to the Cortex A8 CPU (same CPU architecture found in the iPhone 4), and the PowerVR SGX535 GPU (exact same as the one found in the iPhone 4). The processor change alone from 3G to 3GS was somewhere around twice as fast as the one found in the 3G, and the GPU is vastly improved as well. These things are the major contributing factor to how fast and "powerful" the device is, and how fast apps run. That's why many people regard the 3G to 3GS upgrade as the most significant so far. The iPhone 4's CPU/GPU are just a refinement of the one's in the 3GS, mainly refining for size and power consumption. It's why the 3GS benchmarks much closer (and sometimes faster...iPhone 4 has to push more pixels with same GPU because of the Retina display) to the iPhone 4 than the 3G does. Even though the 3G looked just like the 3GS did/does, it was much different. You can think of the 3GS as an "almost" iPhone 4 in a 3G body.