Have Verizon iPhone but AT&T as my carrier?

I lost my phone but my friend has an iPhone 4 that's cleared out and is just like when you get a new iPhone.

The only problem is I have at&t.
Is there any way they can switch it or do I have to buy an at&t iPhone?

There's absolutely nothing you can do - a Verizon (CDMA) iPhone 4 is not capable of running on AT&T.

Hope this helps

Nothing can be done.

Verizon is a CDMA carrier, no SIMs, AT&T is a GSM carrier, SIMs required.