Is it possible to unlock an AT&T IPHONE 4?

-Is it possible for me to unlock (NOT JAILBREAK BUT UNLOCK!! I've ALREADY jailbroken it) an AT&T IPHONE 4 on ios 4.3.3 using a windows computer? My model number is MC608ll if that helps? also my baseband is 04.10.01 and it is to my understanding that you have to first update your baseband to 6.15 so that may be my problem. When i did my 3gs using sn0wbreeze the process went smoothly and i was able to update my baseband off of snowbreeze but when i plug in my iphone 4 it doesnt seem to offer it. I can't figure it out! Does anyone know?? If you do how do i do it?? i cant figure out how to freaking update my baseband!! >.<