I don’t believe that assistant will require the next iPhone. Siri works remarkably well, and does most of the functionality that Assistant seems to have: location awareness and contextual searching, contact recognition, and the usual things that make a phone smart. Yes, the answers may take more time on an A4 chip, but the results should be the same.
And if it doesn’t require new hardware, then consider this a dud release for existing iPhone 4 owners. The iPhone 4 is already incredible fast for my usage and has an excellent display. iOS 5 runs well on it. If we take 9to5’s article to be fact, this upgrade will not be as remarkable as the 3GS was for the 3G.