The main reason most of you want to know how to unlock your iPhone 4 is so that it can be used on any wireless provider of cell phone service. When an iPhone is unlocked, the restrictions on the modem and the SIM card are removed. It is very simple to unlock an iPhone and if you ever have a reason to it can be reversed.
Get online and go to Blackrain.com and then select the operating system you use to sync with the iPhone, either Mac or Windows. Download the software and run it. Plug in the iPhone and click “make it rain.” The software needed to jailbreak the phone will be installed. When prompted, click, “ok” and in just a few minutes the full program will be on your phone and it will be unlocked.
From the iPhone you can now open “blackrain” and install Cydia. These apps are very similar to the ones available in the App Store. After you open the app and download the upgrade the phone is done. The jailbreak and the unlock is completely done and the iPhone can be used on any network. You may still have to change out the SIM card or obtain one from your other phone to make it work properly.