Is I buy an Iphone 4 can I take the sim card out of my old phone and put it in?

If i put my sim card into the iphone can I call using the iphone

Yes, if its a GSM iPhone 4 that is unlocked, or locked to your current wireless service.

If it is the same phone carrier then yes(like t mobile) if they are not the same carries then you will have to unlock it. What phone carrier r u with and wat carrier is the phone?

The simple answer is yes, as long as the service provider is the same as the phone has been locked to. The only issue is that most older phones use the regular Sim-Card, while the iPhone 4 uses a smaller micro-sim. You could google "sim card cut to micro sim" and find instructions on how to cut down your regular sized sim-card to the size of a micro-sim, or you could go to any phone repair shop, and they will cut it down for a low cost. (probably 10-15 dollars).