I understand your point, and it’s a good one, but iPhone 4 isn’t allowed to run Siri because of hardware reasons, not API support. The noise reduction in newer models is a lot better than on iPhone 4. Hackers have managed to get Siri running on iPhone 4, it just doesn’t work as well.

http://appleinsider.com/articles/12/02/05/apple_iphone_4s_a5...