Install Siri GUI on the iPhone 4 and iPod Touch 4:
1. Firstly jailbreak your iOS 5 based device, you can use Redsn0w or Sn0wbreeze.
2. Download the Siri port for your device, Siri GUI, and download AssitantService.Framework.
3. Download iExplorer to browse your iPhone files and folders.
4. Now use the iExplorer to browse your iOS device, and then copy the AssistantServices.framework folder (AssitantService.Framework you downloaded in Step 2) to Root > System > Library > PrivateFrameworks, do this after plugging in your device to your computer via USB.
5. Now transfer all the files you have downloaded (from step 2) from your home screen folder (springboard) to Root > System > Library > CoreServices > Springboard.app on your device.
6. Use iFile from Cydia, open the .plist file for your iOS device from System > Library > CoreServices > Springboard.app. It will be named N81AP for iPod touch 4G and N9x for iPhone 4 (where x may be 0, 1, 2).
7. Add a new property list named assistant to the .plist file and set its class as Boolean while setting its value as YES.
8. Add the following to the .plist file:
9. Respring your iOS device.
Recommended readings about Siri:
How To Improve Siri Dictation On iPhone 4s (Video).