How to Install Siri on Your Jailbroken 'A4' Apple Device Running iOS 6
Unfortunately, we peasants with A4 devices (3GS, 4, 4th Gen iPod touch) didn't get Siri with the iOS 6 update. Seriously, that was the only reason I updated. That and turn-by-turn directions, which also isn't available on my device.
This eventually led me to jailbreaking my iPhone and downgrading (which I unfortunately did not do very well). But if you've successfully jailbroken your old A4 device, then you can get Siri—and it's free.
How to Install Siri on iPhone 4 / iPod touch 4G with SiriPort :
Step 1: Jailbreak your iPhone 4 / iPod touch on iOS 5.0.1.
Step 2: Open Cydia > Manage > Sources > Edit > Add the below source :
Step 3: Go and search for “Siriport” and install it.
Step 4: Reboot your iPhone 4 / iPod touch 4G.
Step 5: Open Safari the open this link or this link (it will download file to your iPhone 4 / iPod touch 4G that contains the Siri Proxy details, there is nothing to do, it will configure settings itself)