Good news and now it is very easy to install siri on the iPhone 4 and iPod Touch 4G. Just follow below steps and your iphone 4 and ipod touch 4g will be running Siri. You need the following source to add to Cydia (follow below sequence)
Yo need the to visit following website to get the certificate (after source is added)
It may happen that site is not opening in your iphone 4. So open the browser on your computer and go the above mentioned site download the certificate, email it to your email configured on your iphone4 and open it from your email.
Your iphone 4 or ipod touch should be jailbroken if not go here to jailbreak. Now Cydia will be available on your spring board.
How install the Siriport :(Follow below steps)
Add the Source to Cydia
Reboot your device
Go to Cydia > Manage > Sources and your source will be in the list “SiriPort.Ru Repo”
tap it and install.
This will take much time be patient and wait until below screen
Install the Certificate from the website or open from your email. Write siriport.ru/1.crt in your Safari address bar and tap Go a new installation window will appear like below tap Install. If site is not accessible you can Click Here to download 1.crt file.
Go to settings and activate Siri, if Siri is not there re-install Siri port.