If the phone is locked, it may mean several things. It may mean that the phone is locked internally, with a locking system that is part of its security feature. The locking mechanism has been configured by the user and applied according to the configuration. The phone lock can be of several variants. The most commonly seen types are screen lock, phone boot lock, and sim lock.
The screen lock is configured and set up by the user. The screen lock protects the phone from being used by other unauthorized user. The screen lock applies right after the phone goes to timeout after several seconds of idle standby. The screen timeout time is also configured by the user. Screen lock can be set up using passwords, pins, or patterns that are applicable for touch screen phones. If you are wondering how to unlock iphone 4s for screen locks with forgotten pins or passwords, there is still a way to bypass the screen lock.
Phone boot lock
Phone boot lock applies after turning on the phone. It will not go to the next screen unless you enter a pin, password, or draw a pattern on the screen. The method of unlocking a phone boot lock may be similar to the screen lock, although other users preferred that they use a different password or pin instead. The phone boot lock can also be the general phone lock. It means that the pin used for the phone lock can be the same one to be keyed in to do factory reset, formatting of phone memory, setting up locator finder, and others.
This requires a 4-digit pin that was set up by the user. It is needed to access the sim and use the network, but is not necessary to use other functions of the phone.
Look here for more details http://iphone4sunlocker.net/