Link to how to install siri on iPhone 4: http://tipsandhacks.blogspot.in/
For those that haven't been following the progress, you'll be delighted to hear that porting Siri to the iPhone 4 and fourth-generation iPod touch is actually a rather easy process. Siri is a voice recognition tool that allows for you to speak to your Apple handset, making it easier to complete everyday tasks.
With Siri, you can ask your iPhone questions, set timers, dictate text messages, solve math equations, control music playback, check the weather, create notes, and more. Unfortunately, Apple released Siri as a feature exclusive to the iPhone 4S. But, thanks to the work of some Russian developers, Siri has been successfully ported to other iOS devices as well...
Siri on your iPhone 4 or iPod touch 4G is all possible thanks to a Cydia package called "SiriPort." You don't need a proxy server and you don't need to have an iPhone 4S nearby, as SiriPort just works seamlessly. Although we will advise using this port at your own risk, several users have been successful in getting Siri running on their iPhone 4 and fourth-generation iPod touch