Originally Posted by Greenkirby21
follow this guide(i just googled it)
after jailbreaking, you should have cydia installed. Go to cydia and search for ultrasnow. Ultrasnow will let you unlock your phone. Make sure you have your tmobile chip in there and everything should be ok.
Yeah, that's how it was done in the past but with Apple making it harder and harder I wasn't sure if it was as easy as that anymore. Also, since the iPhone 4 uses a micro sim card, I wasn't sure if A. T-mobile was using them as well and B. a Gevey type sim chip was also needed or not.