does iPhone4 run 2-time faster than iPhone 3GS since iPhone4's memory is 2-time bigger than iPhone 3GS's one
No. Well.... maybe.
Although the phone may run twice as fast (I don't know, I don't have any iPhones), it is not the addition of memory that makes it run faster. If the phone was running out of memory and had to kill processes, then more memory would make it seem faster.

What really makes phones, (or any computer-like device for that matter) run faster, is a fast processor. The iPhone 4 also has a faster processor than the 3GS.