No 4s cert needed
from CydiaHelp
directions can be found here
http://cydiahelp.com/install-siri-on-iphone-4-3gs-ipad-ipod-touch-4g-3g/
4Siri is NOT a Siri port on your older device. i4Siri is a smart Siri server which needs Siri to be ported on your device first. i4Siri servers now available to the public for free and it doesnt require you to have a registered iPhone 4S.
Of course, Spire needs your device to be on iOS 5. You may face some problems with i4Siri servers due to huge load on the servers but it will work properly by the time.
from CydiaHelp
directions can be found here
http://cydiahelp.com/install-siri-on-iphone-4-3gs-ipad-ipod-touch-4g-3g/