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Technically, the latest version of Bluetooth which is 4.0 comes packed inside the iPhone 4S. This uses very little power as this is the latest Smart Bluetooth technology. Now the earlier bluetooth versions used to consume a peak power of 25mA whereas bluetooth 4.0 consumes a peak power of 15mA.
Another difference is Bluetooth 4.0 does not consume any power if it is idle. This saves a lot of energy. Thus, if you have an iPhone 4S or similar device which comes installed with Bluetooth 4.0, you can very well keep it on and it will not drain your battery.
Here is a table which compares the specification. The second column listed as BLE is the latest Bluetooth 4.0 configuration.
I am also giving you a link below which has further technical specifications and calculations. Bluetooth 4.0 Low Energy
Page 21 will give you a small calculation as well.
In short, the latest bluetooth which you have on your phone will not give you any battery issues when idle. So you can stop worrying about that. But ideally it is recommended to turn off bluetooth when not in use so as to avoid unknown connections and also to save battery (for previous versions).
Hope this helps. If you have any other query please feel free to ask. I will be glad to answer.