Verizon switch to Virgin mobile for iPhone 4
There are 2 telephone systems in the USA. They are named CDMA and GSM.

One with SIM card and the other without SIM card.

Verizon is without SIM card!

So just check with the new provider what system they use and you will know if your phone works.

Major carriers Sprint and Verizon networks mainly use Code Division Multiple Access (CDMA) technology, while AT&amp;T and T-Mobile use Global System for Mobile communication (GSM) technology. All of these carriers also support 3G or higher networks. The network plays a big part in the capabilities your phone will have and, to some extent, its performance.

GSM phones have a SIM card that stores your account information and, if you choose, your phone book. When you switch to a new phone, you can simply remove the card from inside the old phone and insert it into the new one. But you can't use a T-Mobile SIM card in an AT&amp;T phone, or vice versa. Also, GSM phones work more widely across the world than do CDMA models. 

The good news is:
If you have the iPhone 4s
the iPhone 4S is a world phone," it will support both GSM and CDMA networks. Regardless of your carrier, you can be sure that the micro-SIM slot will be there to accept your GSM micro-SIM cards, as well as any CDMA "roaming SIM.""