Reports doing the rounds are that we may see Apple working on Siri for iPhone 4. Electronista have confirmed rumours that there has indeed been some early testing of the iPhone 4 running Siri on a test build of iOS 5. Information at this point is vague, but it looks like the iPhone 4 and fourth-generation iPod Touch are both capable of running the voice command system.
Although it is completely speculation at this point, it would seem that Apple would likely not push for Siri on iPhone 4 as it is a huge selling feature of the 4S model. It is unlikely that a better camera and faster processor would have the same draw.
Other speculation, however, reports that Siri has been limited to the 4S during Apple’s beta testing of the load on Apple’s servers, and that it may indeed find it way to the iPhone 4. It is thought that the iPod Touch may harm the Siri experience due to its inferior microphone.
It would only be prudent to ask Siri these questions for yourself!