I don't think the iPhone 4 is outwith the requirements for Siri, but as with all iPhones there is always one "next gen" feature to sell the new device.
Siri, IF it's baked into iOS 5, will simply be listed in the plist file like everything else. It should be a simple case of either a) opening up the N90.plist and changing the <string>Enable Siri</string> from </false> to </true> b) manually typing the string that enables Siri, found in the N94.plist that will be created in iOS 5 for the 4S, into your N90.plist file, or c) simply obtaining the N94.plist file and renaming it N90.plist and copying it over your existing N90.plist file.