iPhone developer Steven Troughton-Smith managed to port Siri to iPhone 4 as shown in the above video, but Apple is not currently authenticating Siri commands from iPhone 4. This means that you can certainly install Siri on your iPhone 4 but the digital personal assistant will not respond to your inquiries.
Tech bloggers are speculating that Apple may wish to only allow Siri to connect to their servers using the new iPhone 4S even though there is no technical reason that iPhone 4 and iPad 2 should not be able to use the feature. Perhaps Apple will introduce Siri to these devices once the 4S has been available for a few months.
Siri voice command system ported from iPhone 4S to iPhone 4 [9to5 Mac via Gizmodo]