When the new iOS 6 came out a few months ago, I was pumped because I finally would get Siri on my iPhone 4. Having the automated voice assistant readily available on my phone would be cool, even if I probably wouldn't use it all that much.
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.
So, why exactly did I upgrade?
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.
This is the beta version and does have a few bugs, but it is the only actual Siri option for jailbroken devices right now.