iPhone 4 16gb silent mode problem

Hi,

I've a iPhone 4 16gb unlocked purchased from Canada and is still covered under apple care program. My phone keeps switching between audible mode and silent mode when I switch on the silent key.

Since this phone is discontinued and If I take it to a apple store will I get a replacement or will they repair it?

If under replacement, Can I get 4s instead of 4?

Regards.