People have been clamoring for one of the prominent developers in the jailbreak community to get Siri to port to the iPhone 4 or iPod Touch 4th generation. Now, you can get Siri on your iPhone 4 or iPod Touch 4th generation, kind of. You can get the whole user interface on the device, but none of the voice control works. If you are interested, here is the tutorial.

Step 1: Jailbreak your iPhone/iPod Touch on iOS 5 using Redsn0w or Sn0wbreeze.
Step 2: Download files required for the Siri port to iPhone 4 and iPod touch 4G. (You will have to find these files yourself as we wont be able to share these files with you)

Step 3: Using a file explorer for your iOS device, copy over the AssistantServices.framework folder you downloaded in Step 2 to Root > System > Library > PrivateFrameworks after plugging in your device to your computer via USB.

Step 4: Transfer all files from the Springboard folder in Step 2 to Root > System > Library > CoreServices > Springboard.app on your iOS device.

Step 5: Using iFile (you can get it 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).