Finally i have unlocked the iPhone 4s which Tinyan got it from her uncle about half a year ago. Thanks Steve for letting me know this solution. It unlocks a jailbroken iPhone 4 or iPhone 4s using SAM(Subscriber Artificial Module) which is written by Sam Bingner and it is confirmed by the MuscleNerd of iPhone Dev Team.
You can find the unlock procedures in the following 2 posts.
- iPhoneBlogr – Unlock any jailbroken iPhone with the Loktar_Sun SAM unlock method
- Singularity – How to: Unlock your iPhone with SAM
I tried a few times by following the above steps but no luck. Whenever i connect the iPhone 4s to iTunes in the final step, iTunes always returned that the phone cannot be activated.