Can I use an iPhone 4 with my ATT Go Phone Plan?

I was thinking about buying an iPhone 4 from my friend since he just got the new iPhone 5. I was looking around online and I learned that I will need a microsim in order to use the iPhone 4. If I go to the ATT Store will they be able to switch me from a normal go phone sim to a microsim? I do not wish to cut my existing Sim card into a microsim because that's too risky.

I'm asking this question because I do not want to buy a phone and not be able to use it. I will be able to unlock it once I get it, though because I know I have to do that.
Will I still have internet on the iPhone 4 with my go phone plan? Texting?

Thank you

Added (1). I currently have a Go Phone plan with ATT and I do not want to change it to an iPhone plan.

Be aware, first check with AT&T about the model that you buy because not all of them are compatible with all services. Just call customer support and give them the model number and they'll be able to tell you. Or you can look on the AT&T website, maybe open a customer support ticket for the info, too.