I know this is probably not what you want to hear when you're searching for unlocking an iPhone 4 Baseband 04.12.09, but I just want to warn you that I tried to unlock and jailbreak my iPhone 4s and now it has stopped working and is basically completely worthless...
What I did was silly, so please please if you don't know what you're doing, probably best to take it to a shop or professional that can do it for you.
Now I have a cheap old-style Windows phone :(
Anyway, if you are 100% sure you are ready to try and unlock your own phone, then watch this video tutorial on how to unlock an iphone 4 rocking baseband-041209