Can you use an iPhone 4 without a SIM card?

I have an AT&T iPhone 4, iOS 6. I'm upgrading to a 4S. Can I use the SIM card on the 4S as a phone, but keep my 4 and use it as an iPod touch?

No it is required

IPHONE's don't have sim cards. The store upon which you purchase you're new phone can switch the information from one to the other.

The above idiot is an idiot. Yes, you can use your iphone 4 as an iPod touch without a sims card. My friend uses his iphone 3s as his iPod touch and his iphone. 5 as his iphone. Why? I don't know: L