It wont. besides the 8mp camera and A5 processor, Siri is the only stand-out feature on the 4s to keep it selling.
As far as porting it to an iphone4 right now, you would need a friend or family member who has a 4s that you would have access to every 24 hours to refresh the keys. You also run the risk of Apple catching on to the proxy and banning said friend or family members 4s from using Siri ever again.. So if you ask me it's not worth it.
I installed spire just incase anything arises to make it useful, other than that im just going to wait patiently for something else to pop up.. Which im sure will happen eventually.
Ooooh. It's pretty sad that even though Siri can run flawlessly on the iPhone 4, they still won't release it. Probably just like you said, the only selling point for the iPhone 4S and also releasing siri would piss off many iPhone 4S users.