Hello - long time lurker, first time poster here. I currently have an iPhone 4 on AT&T and would like to get a Galaxy S4. However, I want to be able to switch between the two phones as I please so I can have the best of both worlds. If I get an unlocked S4 model GT-I9500, GT-I9505, or SGH-I337, can I just take the micro sim card from my iPhone 4 and use it in the S4 and have everything just work (voice and data)? Will I be able to switch back and forth without calling AT&T? I have a 3G sim card, will that be an issue in the S4?