Siri is no longer exclusive to iPhone 4S.You can run Siri on your iPhone 4 and iPod Touch 4G now. How to do this?
What you need is just a jailbroken iPhone 4 or iPod Touch 4G. By copying the app onto the device, the iPhone 4 can call up Siri and, more important, it can also connect to the Siri servers. Cool?
You can follow these instructions to install the app yourself. For copyright reasons, we are not authorized to post eh contents here.
To make Siri connect to Siri server using iPhone 4, you may follow these instructions.