4S had revolutionary and significantly higher CPU capability and new audio processing hardware.
Was it necessary? Questionable. But none of us are the software/hardware engineers behind it to know.
My friend had Siri on his iPhone 4 with a jailbreak, so the hardware of the 4S was not needed to make it work. It's not heavy 3D-rendered stuff you know, just some overlay that records audio, sends it to a server and then gives the result...