How to unlock an iphone 4 from sprint?

I have an sprint ihpone 4 and i would like to change to t-mobile but i found a problem the ihpone 4 has no sim card slot so i was wondering if there's any other way to unlock and change the carrier network?

The iPhone 4 is CDMA only which basically means Verizon or Sprint in the US. The iPhone 4S was the first Global Ready iPhone made which has a sim card slot. There's absolutely no way to get your Sprint iPhone 4 on T-Mobile. I don't believe you can get a Verizon or Sprint iPhone 4S on a GSM (at&t, T-Mobile, net 10, straight talk) network anyways due to carrier restrictions. Your best bet is to sell your iPhone 4 and buy an at&t iPhone 4 or 4S and have it unlocked.those are GSM phones with a sim card slot.