Re: phone replacement
Apple Care (regardless of which version you had) would be exhausted by now on any iPhone 4, so no, they would not do this for free (unless they made a special case for you and did it complimentary...which they have been known to do if you find the right person on the right day...it's worth a shot if you have one local to you! The biggest thing you have going against you is the fact that they do not stock 4's anymore, so if they don't have any 4's, you'll probably be out of luck because they wouldn't be able to replace it anyways in store, and would have to send it off for repairs).
Has the phone recently been damaged (water specifically)? The headphone jack issue is typical of moisture damage on any iPhone, not just the 4.