Will my iPhone work with tMobile?

Okay so I haven't bought it yet but I found used iPhone 4 for a good deal and it says it is GMS, which is what I need, but it also says its for at&t but I was wondering if I coukld take it to the store and get a new micro simcard and just change phone but not mess with the plan at all. So basically I'm asking will the iPhone still work with tMobile and if it does will I be able to switch phones with ease in the same family plan we have. And also possibly keep the same phone number?

You need to get it unlocked to work on tmobile.

I just went through this. In order for your phone to work with T-mobile it must have a sim card or chip. It is not a SD card. I had lots of phones with sim cards because they hold all the contracts pictures and everything and the sprint store I was in backed me up and stated they were sim cards. Well T-mobile stated that there not. And if you start looking for phones with sim cards on the internet you will find out real quick that they are all very expensive. T-mobiles rates are good but you have to pay $700 to buy the phone to get the rates. If you want to know if your phone has a sim card that T-mobile can use call them because they are the only ones who matter. I was going to switch from sprint to T-mobile but after buy 4 phones for my family it was to expensive. Just so you know some blackberry phone with the same model have sim cards and some don't so before turning off your other service I would visit a T-mobile store to be 100% sure. They are really nice people.