If you’re looking to install Siri on iPhone 4 or iPod Touch 4G than HiSiri is the answer. HiSiri is a jailbreak tweak that allows you to install Siri’s GUI and server connectivity on your iPhone 4. However, HiSiri doesn’t connect Siri to Apple’s servers, but it does connect it to private servers offered by CD-Team.

In order to install HiSiri on your iPhone 4 or iPod Touch 4G you need to jailbreak it on the latest iOS 5.0 or 5.0.1 and add a certain repository thru Cydia.
Follow the tutorial below to install Siri on iPhone 4 / iPod Touch 4G using HiSiri

• Update your iPhone 4 / iPod Touch 4G to iOS 5.0 / 5.0.1 using iTunes
• Jailbreak your iPhone 4 / iPod Touch 4G using one of our tutorials[ jailbreak 5.0 / 5.0.1 ]
• After the jailbreak is complete, tap on Cydia > Manage > Sources > Edit > Add
• Add “http://www.cydia.be” or “http://www.iphone3gsystem.fr/cydia”
• Head back to Cydia and search for HiSiri ( H1Siri ).
• Install Hisiri and reboot your iPhone 4 / iPod Touch 4G
• Once the reboot is complete just tap on Settings > General and turn on Siri

That’s it, you have successfully managed to install Siri on iPhone 4 / 3GS using HiSiri.

More information [ihowto.com]