iOS 8 has this feature. Your 4S is upgradable if you want to go that route.
If you would rather have a third party app, Camera+ is available as a free download until November 16th if you have the Apple Store app and are a U.S. customer but a few people from other parts of the world have had luck with this method as well.
Grab it while you can if you're able, it has timer mode plus tons of other features. I actually am surprised I haven't seen it mentioned here yet.