If sth is up in the mini jack, they could very argue it's your fault... but worth a try, they replaced my iPhone 4 not even a 4S a little over a month ago and I bought mine 4 years ago...
This happened to me a few weeks back. Turned out to be lots of pocket fluff (eww!) which eventually came out with a bit of careful safety pin maneuverage...
Would this cover my iPhone 4s then? Somethings up with the headphone jack, can't plug in headphone, think something's wedged in there