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The iPhone 4S has an updated proximity sensor, allowing the raise to speak function. The iPhone 4 doesn't have this proximity sensor, so it's not possible to use raise to speak.
 
The iPhone 4S's proximity sensor is on whenever the phone is on. As soon as the phone is unlocked, the sensor is active so it knows where it's at in perspective to your face. This is solely for the purpose of raise to speak with Siri.

The iPhone 4 and previous models have a proximity sensor that ONLY turns on during a phone call so it can black out the screen to prevent face-dialing.
 
I wonder if that's one of the reasons for the 4s battery drain if the sensor is continuously firing !
 
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