Q: iphone4 wont let me unlock it

If your phone is locked with a passcode and you have forgotten the passcode, restore it, using the computer you usually sync to and set up a new passcode.

Follow this article: iOS: Forgot passcode or device disabled

If the phone is locked to another carrier and you want to use another SIM, it needs to be unlocked by the carrier, check this article:

iPhone: About unlocking

If you want to set up the phone and the Activation Lock is active, check this article to remove the lock:

Find My iPhone Activation Lock