Have a question about unlocking iPhone 4 to use on T-Mobile

[RDL]

I'm on T-Mobile and looking to buy a used iPhone 4 (NOT 4S) to unlock/jailbreak to use on my network can someone please tell me what I need to look for to assure I can unlock it and use it on T-Mobile.

Will any Modem Firmware and Version be able to be unlocked or do I need to avoid certain ones?

Thanks in advance to anyone who takes the time to reply. Appreciate it!

[iYeow]

All Baseband of iOS 4 can be unlocked by gevey sims.

[RDL]

Thanks. I never messed with Gevey Sims and heard they have signal problems so I'm just going to give up on the iPhone on T-Mobile and finally try a new phone. Getting the HTC Sensation and once iPhone 5 is released if T-Mobile doesn't get it I will finally say EFF T-Mobile and after almost a decade with them move on to Sprint.