Unfortunately there is no legal way to obtain Siri on any device besides the iPhone4S after 5.0.1. Apple did a strange thing in 5.0.1 and provided an unencrypted firmware which allowed chpwn to create an installer package that downloaded the Siri files directly from Apple servers making it a legal way of installing Siri on other devices. This package became known as Spire. Since 5.0.1, Apple has not released an unencrypted firmware again so there is no legal means of distributing the Siri files. If you want to use Siri on your iPhone4 you will have to downgrade to 5.0.1 to install Spire, however, I must warn you that even with Spire installed, you still need to purchase access to a 3rd party Siri server which is getting harder and harder to find these days.
