A developer, Steven Troughton-Smith successfully ported Siri interface to iPhone 4 at the time of iPhone 4S launch. Siri was one of the best things iPhone 4S has got and obviously millions of iPhone 4 owners wanted to have this cool talking buddy.
Troughton-Smith’s hack was good enough to make Siri on iPhone 4 work perfectly except the inability to communicate with Apple’s servers. This was an important piece of jigsaw and he now fixed it. Current status is, Siri is now fully functional on iPhone 4 and iPod Touch as well including ability to communicate with Apple servers.
iPhone 4S jailbreak is needed in order to accomplish the task of porting the proper files for Siri’s servers to connect to an iPhone 4. According Troughton-Smith’s interview with 9To5Mac, its a 20 step process and requires a validation token from the iPhone 4S.