I played with the SP4 at my local MS store for a few minutes, and might be only one that thought that the device itself was not significantly different. Note that my testing does not involve battery life or processing speed.
The biggest thing is that I didn't notice a huge difference in appearance or screen quality. 0.3 inches did not make a difference to me. I thought they could have easily packed more screen into the frame. It was a little thiner and lighter but again nothing notable.
The biggest improvement to be was the keyboard and touchpad. It made a huge difference, but I can buy that for my SP3.
I think if someone was buying a SP for the first time, they would be very impressed with the SP4 but for someone that has a SP3 I don't think the device itself (not keyboard) is a huge improvement.