Troughton-Smith has been able to successfully port Siri to the iPhone 4 along with a full connection to Apple Servers. The video below not only shows the Siri functionality on an iPhone 4, but is in depth and shows a side-to-side comparison against its newer, faster sibling, the iPhone 4S. The port proves that the software can easily be enabled on the older device and hence points a finger at Apple for not doing so.
Smith did use a jailbroken iPhone 4s to port the necessary files to the older iPhone 4 which was also jailbroken. When asked if he would seed Siri as a package on Cydia he replied
No, I could not be a part of that. I have no doubts that others will package this up and distribute it quasi-illegally, or try and sell it to people. I am only interested in the technology and making it work; proving that it works and works well on the iPhone 4 and other devices.
So despite the fact it can be done, Smith will not be the one to enable it on your device. Apple has to man up to the truth and enable it for the users, until some Jailbreak dev who is not afraid of Apple Law-Suits does it anyway.