It’s a common issue with Apple, they put all the new features into their latest products and that feature set gets smaller as you work your way down the product line.
So with iOS 5.0 the iPhone 4S has all the bells and whistles, the 4 has most, 3GS has a few less, and the 3G has joined Match.com looking for someone to love it.
Most of those new features just don’t work on that older silicon.
But there is one recent feature that has iPhone 4 users baffled and feeling cheated. Why does the 4S have Siri, but not the 4?
Well, naturally people point to the A5 chip inside the 4S. Perhaps the A4 cannot support Siri, perhaps Siri requires some mightily beefy processing?
And the answer is no. It’s not a speed issue. Infact Siri can quite happily run on the A4. If you’ve jailbroken your iPhone 4, you can install it for yourself.
No, the real answer is, it doesn’t work very well. Why, you ask?
Because the A4 isn’t a very good listener. No really, the A5 is a much better listener than the A4, because Apple have integrated EarSmart technology from Audience into the chip.
This allows the A5 to cut through the noise and provide the Siri software with a clearer signal to interpret. So when you ask your 4S to “text mum i love you” it sends a lovely SMS to your mother. Ask your 4 to do the same and it will want to know who “mommalove” is.
So the real reason Apple won’t let you have Siri on the 4 is due to their quality control. Siri works great on the 4S, but the Android Army would have teared them apart if they had dropped Siri onto the 4, and the resulting problems owners would have had with it.
However, perhaps its a blessing in disguise that we poor iPhone 4 owners cannot get Siri, a recent viral video reveals that Siri may have a crossed-wire (it’s a fan-film, but excellently made – safe for viewing at work)
Source: CNET
Senile Developer 
The video is truly awesome, told you apples were bad for you. 🙂