One of the big selling points for the iPhone 4S is Siri, which, according to Apple, wasn't suitable for the iPhone 4. You've probably noticed the voice command system isn't on the iOS 5 upgrade for the iPhone 4 already. But could it actually run on an iPhone 4? The answer is a resounding 'yes'. Up to a point anyway.Developer Steven Troughton-Smith, working alongside the 9to5Mac site, has been able to port the iPhone 4S Siri voice command system to the iPhone 4 by installing the iPhone 4S Siri and Springboard files onto an iPhone 4. On the face of it, Siri works just fine on the last-generation iPhone.Siri on the iPhone 4 was able to recognise spoken commands in both the standard Siri view and the keyboard Dictation view and speak back, although Apple isn't authenticating commands to its servers from iPhone 4 hardware. It's not perfect and is quite sluggish, although this is said to be down to a missing driver, rather than hardware issues.Indeed, the tests do show that there is no technical reason for Apple not to allow Siri to run on the iPhone 4, with the A4 processor perfectly able to do the legwork with it. That doesn't mean there are problems with using in on the iPhone 4 – 9to5Mac says it could be due to special microphone support, or because the A5 may allow it to run slightly quicker. There might well have been problems when long-term testing took place at Apple HQ.Either way, it doesn't look like Siri will be heading to the iPhone 4 officially in the near future, if indeed ever. But both the developer and the 9to5Mac site are continuing to work on the port to iron out the initial problems. If they succeed, you do wonder if Apple will come clean with the actual reason for it being an iPhone 4S exclusive. Wouldn't hold your breath on that one though.