OK I had no idea the CMDA-4S had a gsm chipset (seems its a combo set) so my prior posts were only applicable to sprint-cdma-iphone 4.. sorry about that!
After reading around and not finding one person who has gotten sprint to unlock it domestically (it appears they will unlock for international sims) I would guess it's not possible to get sprint to do the right thing and unlock it properly (via apple) like att does.
You might consider emailing apple about it (as that is what it took to get att to unlock their iphones, yes a customer complaint to apple fixed att's policy
In theory yes you should be able to unlock the sim slot with it being jailbroken BUT if you are just getting rid of it check the prices... a clean and clear sprint-iphone4s might get you more money than a pre-jailbroken one.
Many shy away from pre-jailbroken devices because you have no idea what the seller may have pre-installed on your iphone before you got it
Everytime I get a cheap pre-jailbroken iphone I always restore it to factory and then re-jailbreak it if needed.
The one thing you need to keep in mind is that you need to make sure they clear your ESN/MEID (serial number) after you pay the etf before you list it. The problem some have run into in the past (with non iphones) is that even after they paid their final bill and ETF's sprint never bothered to update the ESN/MEID database... so make sure it's clean and clear.
And just to clarify att doesn't charge for factory unlock. Once your contract is met you just request it and eventually (anywhere from a few days to a few weeks) it just unlocks when you sync it with itunes.