To say the iPhone 4 isn't powerful enough is complete rubbish, all the brains behind siri are kept on a server. The phone is simply sending and receiving data. iPhone 3GS could handle this.
Siri had to be 4S only to save Tim Cook's presentation from being a massive disappointment.
Apart from that, Siri has been on the App Store for some time and now has been pulled. That should tell you everything you need to know.