How to use my tmobile sim card with an unlocked at&t iphone 4?

I have a tmobile sim card sense they are my carrier, but I have a at&t unlocked iphone 4 I was wondering if I could use this phone with my tmobile sim card. Also if I could us my tmobile sim card with a iphone 4s as well.

You can use your SIM in any unlocked GSM phone. There are some limitations. Only 2G (EDGE) data and No MMS. Siri and FaceTime only work with Wi-Fi.