Can I activate an iPhone 4 with T-Mobile?

I'm thinking of buying a used iphone 4 and would like to know if it can be activated with T-Mobile I've heard it can but what would i need to do to put it with T-mibile?

You would need to jailbreak it.

I think some people actually have their carrier as tmobile while using an iphone. You could call or chat or just contact the company somehow and ask what you should do. I have at&t so i don't know about tmobile. You could also go to the apple store or a tmobile kiosk somewhere and they should probably know what to do.

You know what you should do? No need to jailbreak it. Just go to AT&T or whichever provider the phone is factory locked to, give them the iPhone's IMEI number and they will factory unlock it for you. After that you can use the phone with any provider. I recently did it for my sister and it worked fine

If the iPhone 4 is unlocked, you can use it on T-Mobile.