I have an iphone 4, if i buy an iphone 5 can i just switch the sim cards and use the iphone 5?

I have an iphone 4, if i buy an iphone 5 can i just switch the sim cards and use the iphone 5. I have att and the iPhone is currently verizon.

I have att and the iPhone is currently verizon.
--I assume you mean that the iPhone 4 is on AT&T & the iPhone 5 you are looking to get is on Verizon.

If that's the case, no. Verizon is a CDMA carrier, don't use SIM cards vs AT&T which is GSM that does.

And the SIM that the iPhone 4 uses is way bigger than the iPhone 5.

If the iPhone 5 is on same network or unlocked then yes you can use it. You just need to cut the SIM card to fit iPhone 5