Originally Posted by HarryT
Only on the 4S. It's not on the 4.
Correct, updating to iOS 6 doesn't get you Siri on the iPhone 4.
Google Search has a voice search feature though, you can put that on your iPhone 4.
Have you updated your iPad? Keeping your devices on the same iOS might be a reason to update your iPhone. There are some enhancements to sharing, mail, and the phone feature itself.