How do I unlock my iPhone-4 so I can use it in Mexico with a Mexican carrier?
Do you mean to Jailbreak your phone, or to unlock it so you can use it on a different network? 

Unlocks currently depend on your firmware version, which you can check by going to settings >> general >> about and looking at the version number. If you can confirm then we could find the right unlock for your phone, supposing it can be unlocked.