I really thought the 4 would have been significantly faster than the 3GS. Looks like it was a bigger speed increase going from a 3G to a 3GS than 3GS to a 4.
Isn't that kind of obvious?
Which is more noticable:
4 seconds vs 2 seconds or
2 seconds vs 1 second
By ratio they're the same about in speed increase, but different by net difference. As technology advances the net differences will decrease.
edit: after watching it, the iPhone 4 is still very noticeably faster (and doesn't crash when attempting to play Assassin's Creed)