That is what I would do.... I sold my 4s 64gb phone for $400 before buying the 5. You will not have any problems selling a 5s unless the 6 is so great to where no one would consider buying a lesser phone. I just made you nervous, right? But really, the 5s is a much better phone than the 4s, so I would upgrade to take full advantage of ios 7. The 5s can be sold pretty easily.