Finally you can install Siri on iPhone 4, iPhone 3GS and iPod touch 4G running iOS 5 firmware. If your iPhone 4, iPhone 3GS or iPod touch 4G is running iOS 5 firmware, follow this detailed guide to learn how to install Siri.
Do keep in mind that this guide lets you install Siri GUI only on your iPhone 4, iPhone 3GS or iPod touch 4G. The developers have not been able to make Siri on non-iPhone 4S devices connect to Apple's servers which recognize your voice commands and enable Siri to do the tasks you ask it to do.
As mentioned, you'll just be able to get Siri GUI working on your iPhone 4, iPhone 3GS or iPod touch 4G running iOS 5 firmware. If you're not concerned about Siri's functionality, do try out this procedure. Do keep in mind that you yourself will be responsible should anything go wrong. If you're not accustomed to doing complex processes on your iOS devices, its advised that you do not try this.
How To Install Siri On iPhone 4, iPhone 3GS Or iPod touch 4G:
Step 1: Jailbreak iOS 5 firmware on your device using redsn0w
Step 2: Download files required for installing Siri from here. (mirror)
Step 3: Plug in your device to the computer and use software such as iPhone Explorer or i-Funbox to copy AssistantServices.framework folder downloaded from the link in Step 2 to this location on your device: Root>System>Library>PrivateFrameworks
Step 4: Open Springboard folder from the files downloaded in Step 2 and copy all of the files to this location on your device: Root>System>Library>CoreServices>Springboard.app
Step 5: Now download iFile from Cydia if you don't already have it and go to this location: System>Library>CoreServices>Springboard.app, from here open the .plist file for your respective device. For iPhone 4 the file will either be named N90, N91 or N92 and for iPod touch it will be named N81AP
Step 6: Add new property list to the .plist file and name it assistant. Set class to Boolean and value as Yes
Step 7: Add this: <key>assistant</key><true/> to the .plist file
After adding this, simply re-spring your device. You have successfully installed Siri on your iPhone 4, iPhone 3GS or iPod touch 4G. Once again keep in mind that Siri on your device won't be able to perform any functions as its unable to connect to Apple's servers. You just get the GUI after performing this procedure to install Siri on iPhone 4, iPhone 3GS or iPod touch 4G.
If you face any problems during the installation process, simply leave a comment down below and we will help you out.