How do I turn off camera shutter sound on iPhone 4s

You don't.

If there were a setting for this it would be available at Settings > Sounds.

Well, you can use the mute switch on the left side of your phone, if you don't like the sound, but that will also mute all sounds.

i did used the mute switch but still the sound occurs when taking a photo..what should i do to mute the shutter sound?...

Something is wrong with your iPhone in that case since the mute switch when on mutes the camera shutter sound when capturing a photo.